path: root/repository/git.go

// Package repository contains helper methods for working with the Git repo.
package repository

import (
	"bytes"
	"crypto/sha1"
	"fmt"
	"github.com/MichaelMure/git-bug/util"
	"io"
	"os"
	"os/exec"
	"strings"
)

const branchRefPrefix = "refs/heads/"

// GitRepo represents an instance of a (local) git repository.
type GitRepo struct {
	Path string
}

// Run the given git command with the given I/O reader/writers, returning an error if it fails.
func (repo *GitRepo) runGitCommandWithIO(stdin io.Reader, stdout, stderr io.Writer, args ...string) error {
	cmd := exec.Command("git", args...)
	cmd.Dir = repo.Path
	cmd.Stdin = stdin
	cmd.Stdout = stdout
	cmd.Stderr = stderr
	return cmd.Run()
}

// Run the given git command and return its stdout, or an error if the command fails.
func (repo *GitRepo) runGitCommandRaw(args ...string) (string, string, error) {
	var stdout bytes.Buffer
	var stderr bytes.Buffer
	err := repo.runGitCommandWithIO(nil, &stdout, &stderr, args...)
	return strings.TrimSpace(stdout.String()), strings.TrimSpace(stderr.String()), err
}

// Run the given git command and return its stdout, or an error if the command fails.
func (repo *GitRepo) runGitCommand(args ...string) (string, error) {
	stdout, stderr, err := repo.runGitCommandRaw(args...)
	if err != nil {
		if stderr == "" {
			stderr = "Error running git command: " + strings.Join(args, " ")
		}
		err = fmt.Errorf(stderr)
	}
	return stdout, err
}

// Run the given git command using the same stdin, stdout, and stderr as the review tool.
func (repo *GitRepo) runGitCommandInline(args ...string) error {
	return repo.runGitCommandWithIO(os.Stdin, os.Stdout, os.Stderr, args...)
}

// NewGitRepo determines if the given working directory is inside of a git repository,
// and returns the corresponding GitRepo instance if it is.
func NewGitRepo(path string) (*GitRepo, error) {
	repo := &GitRepo{Path: path}
	_, _, err := repo.runGitCommandRaw("rev-parse")
	if err == nil {
		return repo, nil
	}
	if _, ok := err.(*exec.ExitError); ok {
		return nil, err
	}
	return nil, err
}

// GetPath returns the path to the repo.
func (repo *GitRepo) GetPath() string {
	return repo.Path
}

// GetRepoStateHash returns a hash which embodies the entire current state of a repository.
func (repo *GitRepo) GetRepoStateHash() (string, error) {
	stateSummary, err := repo.runGitCommand("show-ref")
	return fmt.Sprintf("%x", sha1.Sum([]byte(stateSummary))), err
}

// GetUserName returns the name the the user has used to configure git
func (repo *GitRepo) GetUserName() (string, error) {
	return repo.runGitCommand("config", "user.name")
}

// GetUserEmail returns the email address that the user has used to configure git.
func (repo *GitRepo) GetUserEmail() (string, error) {
	return repo.runGitCommand("config", "user.email")
}

// GetCoreEditor returns the name of the editor that the user has used to configure git.
func (repo *GitRepo) GetCoreEditor() (string, error) {
	return repo.runGitCommand("var", "GIT_EDITOR")
}

// PullRefs pull git refs from a remote
func (repo *GitRepo) PullRefs(remote string, refPattern string) error {
	fetchRefSpec := fmt.Sprintf("+%s:%s", refPattern, refPattern)
	err := repo.runGitCommandInline("fetch", remote, fetchRefSpec)

	// TODO: merge new data

	return err
}

// PushRefs push git refs to a remote
func (repo *GitRepo) PushRefs(remote string, refPattern string) error {
	// The push is liable to fail if the user forgot to do a pull first, so
	// we treat errors as user errors rather than fatal errors.
	err := repo.runGitCommandInline("push", remote, refPattern)
	if err != nil {
		return fmt.Errorf("failed to push to the remote '%s': %v", remote, err)
	}
	return nil
}

// StoreData will store arbitrary data and return the corresponding hash
func (repo *GitRepo) StoreData(data []byte) (util.Hash, error) {
	var stdin = bytes.NewReader(data)
	var stdout bytes.Buffer
	var stderr bytes.Buffer

	err := repo.runGitCommandWithIO(stdin, &stdout, &stderr, "hash-object", "--stdin", "-w")

	return util.Hash(stdout.String()), err
}

/*
//
//// GetSubmitStrategy returns the way in which a review is submitted
//func (repo *GitRepo) GetSubmitStrategy() (string, error) {
//	submitStrategy, _ := repo.runGitCommand("config", "appraise.submit")
//	return submitStrategy, nil
//}
//
//// HasUncommittedChanges returns true if there are local, uncommitted changes.
//func (repo *GitRepo) HasUncommittedChanges() (bool, error) {
//	out, err := repo.runGitCommand("status", "--porcelain")
//	if err != nil {
//		return false, err
//	}
//	if len(out) > 0 {
//		return true, nil
//	}
//	return false, nil
//}
//
//// VerifyCommit verifies that the supplied hash points to a known commit.
//func (repo *GitRepo) VerifyCommit(hash string) error {
//	out, err := repo.runGitCommand("cat-file", "-t", hash)
//	if err != nil {
//		return err
//	}
//	objectType := strings.TrimSpace(string(out))
//	if objectType != "commit" {
//		return fmt.Errorf("Hash %q points to a non-commit object of type %q", hash, objectType)
//	}
//	return nil
//}
//
//// VerifyGitRef verifies that the supplied ref points to a known commit.
//func (repo *GitRepo) VerifyGitRef(ref string) error {
//	_, err := repo.runGitCommand("show-ref", "--verify", ref)
//	return err
//}
//
//// GetHeadRef returns the ref that is the current HEAD.
//func (repo *GitRepo) GetHeadRef() (string, error) {
//	return repo.runGitCommand("symbolic-ref", "HEAD")
//}
//
//// GetCommitHash returns the hash of the commit pointed to by the given ref.
//func (repo *GitRepo) GetCommitHash(ref string) (string, error) {
//	return repo.runGitCommand("show", "-s", "--format=%H", ref)
//}
//
//// ResolveRefCommit returns the commit pointed to by the given ref, which may be a remote ref.
////
//// This differs from GetCommitHash which only works on exact matches, in that it will try to
//// intelligently handle the scenario of a ref not existing locally, but being known to exist
//// in a remote repo.
////
//// This method should be used when a command may be performed by either the reviewer or the
//// reviewee, while GetCommitHash should be used when the encompassing command should only be
//// performed by the reviewee.
//func (repo *GitRepo) ResolveRefCommit(ref string) (string, error) {
//	if err := repo.VerifyGitRef(ref); err == nil {
//		return repo.GetCommitHash(ref)
//	}
//	if strings.HasPrefix(ref, "refs/heads/") {
//		// The ref is a branch. Check if it exists in exactly one remote
//		pattern := strings.Replace(ref, "refs/heads", "**", 1)
//		matchingOutput, err := repo.runGitCommand("for-each-ref", "--format=%(refname)", pattern)
//		if err != nil {
//			return "", err
//		}
//		matchingRefs := strings.Split(matchingOutput, "\n")
//		if len(matchingRefs) == 1 && matchingRefs[0] != "" {
//			// There is exactly one match
//			return repo.GetCommitHash(matchingRefs[0])
//		}
//		return "", fmt.Errorf("Unable to find a git ref matching the pattern %q", pattern)
//	}
//	return "", fmt.Errorf("Unknown git ref %q", ref)
//}
//
//// GetCommitMessage returns the message stored in the commit pointed to by the given ref.
//func (repo *GitRepo) GetCommitMessage(ref string) (string, error) {
//	return repo.runGitCommand("show", "-s", "--format=%B", ref)
//}
//
//// GetCommitTime returns the commit time of the commit pointed to by the given ref.
//func (repo *GitRepo) GetCommitTime(ref string) (string, error) {
//	return repo.runGitCommand("show", "-s", "--format=%ct", ref)
//}
//
//// GetLastParent returns the last parent of the given commit (as ordered by git).
//func (repo *GitRepo) GetLastParent(ref string) (string, error) {
//	return repo.runGitCommand("rev-list", "--skip", "1", "-n", "1", ref)
//}
//
//// GetCommitDetails returns the details of a commit's metadata.
//func (repo GitRepo) GetCommitDetails(ref string) (*CommitDetails, error) {
//	var err error
//	show := func(formatString string) (result string) {
//		if err != nil {
//			return ""
//		}
//		result, err = repo.runGitCommand("show", "-s", ref, fmt.Sprintf("--format=tformat:%s", formatString))
//		return result
//	}
//
//	jsonFormatString := "{\"tree\":\"%T\", \"time\": \"%at\"}"
//	detailsJSON := show(jsonFormatString)
//	if err != nil {
//		return nil, err
//	}
//	var details CommitDetails
//	err = json.Unmarshal([]byte(detailsJSON), &details)
//	if err != nil {
//		return nil, err
//	}
//	details.Author = show("%an")
//	details.AuthorEmail = show("%ae")
//	details.Summary = show("%s")
//	parentsString := show("%P")
//	details.Parents = strings.Split(parentsString, " ")
//	if err != nil {
//		return nil, err
//	}
//	return &details, nil
//}
//
//// MergeBase determines if the first commit that is an ancestor of the two arguments.
//func (repo *GitRepo) MergeBase(a, b string) (string, error) {
//	return repo.runGitCommand("merge-base", a, b)
//}
//
//// IsAncestor determines if the first argument points to a commit that is an ancestor of the second.
//func (repo *GitRepo) IsAncestor(ancestor, descendant string) (bool, error) {
//	_, _, err := repo.runGitCommandRaw("merge-base", "--is-ancestor", ancestor, descendant)
//	if err == nil {
//		return true, nil
//	}
//	if _, ok := err.(*exec.ExitError); ok {
//		return false, nil
//	}
//	return false, fmt.Errorf("Error while trying to determine commit ancestry: %v", err)
//}
//
//// Diff computes the diff between two given commits.
//func (repo *GitRepo) Diff(left, right string, diffArgs ...string) (string, error) {
//	args := []string{"diff"}
//	args = append(args, diffArgs...)
//	args = append(args, fmt.Sprintf("%s..%s", left, right))
//	return repo.runGitCommand(args...)
//}
//
//// Show returns the contents of the given file at the given commit.
//func (repo *GitRepo) Show(commit, path string) (string, error) {
//	return repo.runGitCommand("show", fmt.Sprintf("%s:%s", commit, path))
//}
//
//// SwitchToRef changes the currently-checked-out ref.
//func (repo *GitRepo) SwitchToRef(ref string) error {
//	// If the ref starts with "refs/heads/", then we have to trim that prefix,
//	// or else we will wind up in a detached HEAD state.
//	if strings.HasPrefix(ref, branchRefPrefix) {
//		ref = ref[len(branchRefPrefix):]
//	}
//	_, err := repo.runGitCommand("checkout", ref)
//	return err
//}
//
//// mergeArchives merges two archive refs.
//func (repo *GitRepo) mergeArchives(archive, remoteArchive string) error {
//	remoteHash, err := repo.GetCommitHash(remoteArchive)
//	if err != nil {
//		return err
//	}
//	if remoteHash == "" {
//		// The remote archive does not exist, so we have nothing to do
//		return nil
//	}
//
//	archiveHash, err := repo.GetCommitHash(archive)
//	if err != nil {
//		return err
//	}
//	if archiveHash == "" {
//		// The local archive does not exist, so we merely need to set it
//		_, err := repo.runGitCommand("update-ref", archive, remoteHash)
//		return err
//	}
//
//	isAncestor, err := repo.IsAncestor(archiveHash, remoteHash)
//	if err != nil {
//		return err
//	}
//	if isAncestor {
//		// The archive can simply be fast-forwarded
//		_, err := repo.runGitCommand("update-ref", archive, remoteHash, archiveHash)
//		return err
//	}
//
//	// Create a merge commit of the two archives
//	refDetails, err := repo.GetCommitDetails(remoteArchive)
//	if err != nil {
//		return err
//	}
//	newArchiveHash, err := repo.runGitCommand("commit-tree", "-p", remoteHash, "-p", archiveHash, "-m", "Merge local and remote archives", refDetails.Tree)
//	if err != nil {
//		return err
//	}
//	newArchiveHash = strings.TrimSpace(newArchiveHash)
//	_, err = repo.runGitCommand("update-ref", archive, newArchiveHash, archiveHash)
//	return err
//}
//
//// ArchiveRef adds the current commit pointed to by the 'ref' argument
//// under the ref specified in the 'archive' argument.
////
//// Both the 'ref' and 'archive' arguments are expected to be the fully
//// qualified names of git refs (e.g. 'refs/heads/my-change' or
//// 'refs/devtools/archives/reviews').
////
//// If the ref pointed to by the 'archive' argument does not exist
//// yet, then it will be created.
//func (repo *GitRepo) ArchiveRef(ref, archive string) error {
//	refHash, err := repo.GetCommitHash(ref)
//	if err != nil {
//		return err
//	}
//	refDetails, err := repo.GetCommitDetails(ref)
//	if err != nil {
//		return err
//	}
//
//	commitTreeArgs := []string{"commit-tree"}
//	archiveHash, err := repo.GetCommitHash(archive)
//	if err != nil {
//		archiveHash = ""
//	} else {
//		commitTreeArgs = append(commitTreeArgs, "-p", archiveHash)
//	}
//	commitTreeArgs = append(commitTreeArgs, "-p", refHash, "-m", fmt.Sprintf("Archive %s", refHash), refDetails.Tree)
//	newArchiveHash, err := repo.runGitCommand(commitTreeArgs...)
//	if err != nil {
//		return err
//	}
//	newArchiveHash = strings.TrimSpace(newArchiveHash)
//	updateRefArgs := []string{"update-ref", archive, newArchiveHash}
//	if archiveHash != "" {
//		updateRefArgs = append(updateRefArgs, archiveHash)
//	}
//	_, err = repo.runGitCommand(updateRefArgs...)
//	return err
//}
//
//// MergeRef merges the given ref into the current one.
////
//// The ref argument is the ref to merge, and fastForward indicates that the
//// current ref should only move forward, as opposed to creating a bubble merge.
//// The messages argument(s) provide text that should be included in the default
//// merge commit message (separated by blank lines).
//func (repo *GitRepo) MergeRef(ref string, fastForward bool, messages ...string) error {
//	args := []string{"merge"}
//	if fastForward {
//		args = append(args, "--ff", "--ff-only")
//	} else {
//		args = append(args, "--no-ff")
//	}
//	if len(messages) > 0 {
//		commitMessage := strings.Join(messages, "\n\n")
//		args = append(args, "-e", "-m", commitMessage)
//	}
//	args = append(args, ref)
//	return repo.runGitCommandInline(args...)
//}
//
//// RebaseRef rebases the current ref onto the given one.
//func (repo *GitRepo) RebaseRef(ref string) error {
//	return repo.runGitCommandInline("rebase", "-i", ref)
//}
//
//// ListCommits returns the list of commits reachable from the given ref.
////
//// The generated list is in chronological order (with the oldest commit first).
////
//// If the specified ref does not exist, then this method returns an empty result.
//func (repo *GitRepo) ListCommits(ref string) []string {
//	var stdout bytes.Buffer
//	var stderr bytes.Buffer
//	if err := repo.runGitCommandWithIO(nil, &stdout, &stderr, "rev-list", "--reverse", ref); err != nil {
//		return nil
//	}
//
//	byteLines := bytes.Split(stdout.Bytes(), []byte("\n"))
//	var commits []string
//	for _, byteLine := range byteLines {
//		commits = append(commits, string(byteLine))
//	}
//	return commits
//}
//
//// ListCommitsBetween returns the list of commits between the two given revisions.
////
//// The "from" parameter is the starting point (exclusive), and the "to"
//// parameter is the ending point (inclusive).
////
//// The "from" commit does not need to be an ancestor of the "to" commit. If it
//// is not, then the merge base of the two is used as the starting point.
//// Admittedly, this makes calling these the "between" commits is a bit of a
//// misnomer, but it also makes the method easier to use when you want to
//// generate the list of changes in a feature branch, as it eliminates the need
//// to explicitly calculate the merge base. This also makes the semantics of the
//// method compatible with git's built-in "rev-list" command.
////
//// The generated list is in chronological order (with the oldest commit first).
//func (repo *GitRepo) ListCommitsBetween(from, to string) ([]string, error) {
//	out, err := repo.runGitCommand("rev-list", "--reverse", from+".."+to)
//	if err != nil {
//		return nil, err
//	}
//	if out == "" {
//		return nil, nil
//	}
//	return strings.Split(out, "\n"), nil
//}
//
//// GetNotes uses the "git" command-line tool to read the notes from the given ref for a given revision.
//func (repo *GitRepo) GetNotes(notesRef, revision string) []Note {
//	var notes []Note
//	rawNotes, err := repo.runGitCommand("notes", "--ref", notesRef, "show", revision)
//	if err != nil {
//		// We just assume that this means there are no notes
//		return nil
//	}
//	for _, line := range strings.Split(rawNotes, "\n") {
//		notes = append(notes, Note([]byte(line)))
//	}
//	return notes
//}
//
//func stringsReader(s []*string) io.Reader {
//	var subReaders []io.Reader
//	for _, strPtr := range s {
//		subReader := strings.NewReader(*strPtr)
//		subReaders = append(subReaders, subReader, strings.NewReader("\n"))
//	}
//	return io.MultiReader(subReaders...)
//}
//
//// splitBatchCheckOutput parses the output of a 'git cat-file --batch-check=...' command.
////
//// The output is expected to be formatted as a series of entries, with each
//// entry consisting of:
//// 1. The SHA1 hash of the git object being output, followed by a space.
//// 2. The git "type" of the object (commit, blob, tree, missing, etc), followed by a newline.
////
//// To generate this format, make sure that the 'git cat-file' command includes
//// the argument '--batch-check=%(objectname) %(objecttype)'.
////
//// The return value is a map from object hash to a boolean indicating if that object is a commit.
//func splitBatchCheckOutput(out *bytes.Buffer) (map[string]bool, error) {
//	isCommit := make(map[string]bool)
//	reader := bufio.NewReader(out)
//	for {
//		nameLine, err := reader.ReadString(byte(' '))
//		if err == io.EOF {
//			return isCommit, nil
//		}
//		if err != nil {
//			return nil, fmt.Errorf("Failure while reading the next object name: %v", err)
//		}
//		nameLine = strings.TrimSuffix(nameLine, " ")
//		typeLine, err := reader.ReadString(byte('\n'))
//		if err != nil && err != io.EOF {
//			return nil, fmt.Errorf("Failure while reading the next object type: %q - %v", nameLine, err)
//		}
//		typeLine = strings.TrimSuffix(typeLine, "\n")
//		if typeLine == "commit" {
//			isCommit[nameLine] = true
//		}
//	}
//}
//
//// splitBatchCatFileOutput parses the output of a 'git cat-file --batch=...' command.
////
//// The output is expected to be formatted as a series of entries, with each
//// entry consisting of:
//// 1. The SHA1 hash of the git object being output, followed by a newline.
//// 2. The size of the object's contents in bytes, followed by a newline.
//// 3. The objects contents.
////
//// To generate this format, make sure that the 'git cat-file' command includes
//// the argument '--batch=%(objectname)\n%(objectsize)'.
//func splitBatchCatFileOutput(out *bytes.Buffer) (map[string][]byte, error) {
//	contentsMap := make(map[string][]byte)
//	reader := bufio.NewReader(out)
//	for {
//		nameLine, err := reader.ReadString(byte('\n'))
//		if strings.HasSuffix(nameLine, "\n") {
//			nameLine = strings.TrimSuffix(nameLine, "\n")
//		}
//		if err == io.EOF {
//			return contentsMap, nil
//		}
//		if err != nil {
//			return nil, fmt.Errorf("Failure while reading the next object name: %v", err)
//		}
//		sizeLine, err := reader.ReadString(byte('\n'))
//		if strings.HasSuffix(sizeLine, "\n") {
//			sizeLine = strings.TrimSuffix(sizeLine, "\n")
//		}
//		if err != nil {
//			return nil, fmt.Errorf("Failure while reading the next object size: %q - %v", nameLine, err)
//		}
//		size, err := strconv.Atoi(sizeLine)
//		if err != nil {
//			return nil, fmt.Errorf("Failure while parsing the next object size: %q - %v", nameLine, err)
//		}
//		contentBytes := make([]byte, size, size)
//		readDest := contentBytes
//		len := 0
//		err = nil
//		for err == nil && len < size {
//			nextLen := 0
//			nextLen, err = reader.Read(readDest)
//			len += nextLen
//			readDest = contentBytes[len:]
//		}
//		contentsMap[nameLine] = contentBytes
//		if err == io.EOF {
//			return contentsMap, nil
//		}
//		if err != nil {
//			return nil, err
//		}
//		for bs, err := reader.Peek(1); err == nil && bs[0] == byte('\n'); bs, err = reader.Peek(1) {
//			reader.ReadByte()
//		}
//	}
//}
//
//// notesMapping represents the association between a git object and the notes for that object.
//type notesMapping struct {
//	ObjectHash *string
//	NotesHash  *string
//}
//
//// notesOverview represents a high-level overview of all the notes under a single notes ref.
//type notesOverview struct {
//	NotesMappings      []*notesMapping
//	ObjectHashesReader io.Reader
//	NotesHashesReader  io.Reader
//}
//
//// notesOverview returns an overview of the git notes stored under the given ref.
//func (repo *GitRepo) notesOverview(notesRef string) (*notesOverview, error) {
//	var stdout bytes.Buffer
//	var stderr bytes.Buffer
//	if err := repo.runGitCommandWithIO(nil, &stdout, &stderr, "notes", "--ref", notesRef, "list"); err != nil {
//		return nil, err
//	}
//
//	var notesMappings []*notesMapping
//	var objHashes []*string
//	var notesHashes []*string
//	outScanner := bufio.NewScanner(&stdout)
//	for outScanner.Scan() {
//		line := outScanner.Text()
//		lineParts := strings.Split(line, " ")
//		if len(lineParts) != 2 {
//			return nil, fmt.Errorf("Malformed output line from 'git-notes list': %q", line)
//		}
//		objHash := &lineParts[1]
//		notesHash := &lineParts[0]
//		notesMappings = append(notesMappings, &notesMapping{
//			ObjectHash: objHash,
//			NotesHash:  notesHash,
//		})
//		objHashes = append(objHashes, objHash)
//		notesHashes = append(notesHashes, notesHash)
//	}
//	err := outScanner.Err()
//	if err != nil && err != io.EOF {
//		return nil, fmt.Errorf("Failure parsing the output of 'git-notes list': %v", err)
//	}
//	return &notesOverview{
//		NotesMappings:      notesMappings,
//		ObjectHashesReader: stringsReader(objHashes),
//		NotesHashesReader:  stringsReader(notesHashes),
//	}, nil
//}
//
//// getIsCommitMap returns a mapping of all the annotated objects that are commits.
//func (overview *notesOverview) getIsCommitMap(repo *GitRepo) (map[string]bool, error) {
//	var stdout bytes.Buffer
//	var stderr bytes.Buffer
//	if err := repo.runGitCommandWithIO(overview.ObjectHashesReader, &stdout, &stderr, "cat-file", "--batch-check=%(objectname) %(objecttype)"); err != nil {
//		return nil, fmt.Errorf("Failure performing a batch file check: %v", err)
//	}
//	isCommit, err := splitBatchCheckOutput(&stdout)
//	if err != nil {
//		return nil, fmt.Errorf("Failure parsing the output of a batch file check: %v", err)
//	}
//	return isCommit, nil
//}
//
//// getNoteContentsMap returns a mapping from all the notes hashes to their contents.
//func (overview *notesOverview) getNoteContentsMap(repo *GitRepo) (map[string][]byte, error) {
//	var stdout bytes.Buffer
//	var stderr bytes.Buffer
//	if err := repo.runGitCommandWithIO(overview.NotesHashesReader, &stdout, &stderr, "cat-file", "--batch=%(objectname)\n%(objectsize)"); err != nil {
//		return nil, fmt.Errorf("Failure performing a batch file read: %v", err)
//	}
//	noteContentsMap, err := splitBatchCatFileOutput(&stdout)
//	if err != nil {
//		return nil, fmt.Errorf("Failure parsing the output of a batch file read: %v", err)
//	}
//	return noteContentsMap, nil
//}
//
//// GetAllNotes reads the contents of the notes under the given ref for every commit.
////
//// The returned value is a mapping from commit hash to the list of notes for that commit.
////
//// This is the batch version of the corresponding GetNotes(...) method.
//func (repo *GitRepo) GetAllNotes(notesRef string) (map[string][]Note, error) {
//	// This code is unfortunately quite complicated, but it needs to be so.
//	//
//	// Conceptually, this is equivalent to:
//	//   result := make(map[string][]Note)
//	//   for _, commit := range repo.ListNotedRevisions(notesRef) {
//	//     result[commit] = repo.GetNotes(notesRef, commit)
//	//   }
//	//   return result, nil
//	//
//	// However, that logic would require separate executions of the 'git'
//	// command for every annotated commit. For a repo with 10s of thousands
//	// of reviews, that would mean calling Cmd.Run(...) 10s of thousands of
//	// times. That, in turn, would take so long that the tool would be unusable.
//	//
//	// This method avoids that by taking advantage of the 'git cat-file --batch="..."'
//	// command. That allows us to use a single invocation of Cmd.Run(...) to
//	// inspect multiple git objects at once.
//	//
//	// As such, regardless of the number of reviews in a repo, we can get all
//	// of the notes using a total of three invocations of Cmd.Run(...):
//	//  1. One to list all the annotated objects (and their notes hash)
//	//  2. A second one to filter out all of the annotated objects that are not commits.
//	//  3. A final one to get the contents of all of the notes blobs.
//	overview, err := repo.notesOverview(notesRef)
//	if err != nil {
//		return nil, err
//	}
//	isCommit, err := overview.getIsCommitMap(repo)
//	if err != nil {
//		return nil, fmt.Errorf("Failure building the set of commit objects: %v", err)
//	}
//	noteContentsMap, err := overview.getNoteContentsMap(repo)
//	if err != nil {
//		return nil, fmt.Errorf("Failure building the mapping from notes hash to contents: %v", err)
//	}
//	commitNotesMap := make(map[string][]Note)
//	for _, notesMapping := range overview.NotesMappings {
//		if !isCommit[*notesMapping.ObjectHash] {
//			continue
//		}
//		noteBytes := noteContentsMap[*notesMapping.NotesHash]
//		byteSlices := bytes.Split(noteBytes, []byte("\n"))
//		var notes []Note
//		for _, slice := range byteSlices {
//			notes = append(notes, Note(slice))
//		}
//		commitNotesMap[*notesMapping.ObjectHash] = notes
//	}
//
//	return commitNotesMap, nil
//}
//
//// AppendNote appends a note to a revision under the given ref.
//func (repo *GitRepo) AppendNote(notesRef, revision string, note Note) error {
//	_, err := repo.runGitCommand("notes", "--ref", notesRef, "append", "-m", string(note), revision)
//	return err
//}
//
//// ListNotedRevisions returns the collection of revisions that are annotated by notes in the given ref.
//func (repo *GitRepo) ListNotedRevisions(notesRef string) []string {
//	var revisions []string
//	notesListOut, err := repo.runGitCommand("notes", "--ref", notesRef, "list")
//	if err != nil {
//		return nil
//	}
//	notesList := strings.Split(notesListOut, "\n")
//	for _, notePair := range notesList {
//		noteParts := strings.SplitN(notePair, " ", 2)
//		if len(noteParts) == 2 {
//			objHash := noteParts[1]
//			objType, err := repo.runGitCommand("cat-file", "-t", objHash)
//			// If a note points to an object that we do not know about (yet), then err will not
//			// be nil. We can safely just ignore those notes.
//			if err == nil && objType == "commit" {
//				revisions = append(revisions, objHash)
//			}
//		}
//	}
//	return revisions
//}
//
//// PushNotes pushes git notes to a remote repo.
//func (repo *GitRepo) PushNotes(remote, notesRefPattern string) error {
//	refspec := fmt.Sprintf("%s:%s", notesRefPattern, notesRefPattern)
//
//	// The push is liable to fail if the user forgot to do a pull first, so
//	// we treat errors as user errors rather than fatal errors.
//	err := repo.runGitCommandInline("push", remote, refspec)
//	if err != nil {
//		return fmt.Errorf("Failed to push to the remote '%s': %v", remote, err)
//	}
//	return nil
//}
//
//// PushNotesAndArchive pushes the given notes and archive refs to a remote repo.
//func (repo *GitRepo) PushNotesAndArchive(remote, notesRefPattern, archiveRefPattern string) error {
//	notesRefspec := fmt.Sprintf("%s:%s", notesRefPattern, notesRefPattern)
//	archiveRefspec := fmt.Sprintf("%s:%s", archiveRefPattern, archiveRefPattern)
//	err := repo.runGitCommandInline("push", remote, notesRefspec, archiveRefspec)
//	if err != nil {
//		return fmt.Errorf("Failed to push the local archive to the remote '%s': %v", remote, err)
//	}
//	return nil
//}
//
//func getRemoteNotesRef(remote, localNotesRef string) string {
//	relativeNotesRef := strings.TrimPrefix(localNotesRef, "refs/notes/")
//	return "refs/notes/" + remote + "/" + relativeNotesRef
//}
//
//func (repo *GitRepo) mergeRemoteNotes(remote, notesRefPattern string) error {
//	remoteRefs, err := repo.runGitCommand("ls-remote", remote, notesRefPattern)
//	if err != nil {
//		return err
//	}
//	for _, line := range strings.Split(remoteRefs, "\n") {
//		lineParts := strings.Split(line, "\t")
//		if len(lineParts) == 2 {
//			ref := lineParts[1]
//			remoteRef := getRemoteNotesRef(remote, ref)
//			_, err := repo.runGitCommand("notes", "--ref", ref, "merge", remoteRef, "-s", "cat_sort_uniq")
//			if err != nil {
//				return err
//			}
//		}
//	}
//	return nil
//}
//
//// PullNotes fetches the contents of the given notes ref from a remote repo,
//// and then merges them with the corresponding local notes using the
//// "cat_sort_uniq" strategy.
//func (repo *GitRepo) PullNotes(remote, notesRefPattern string) error {
//	remoteNotesRefPattern := getRemoteNotesRef(remote, notesRefPattern)
//	fetchRefSpec := fmt.Sprintf("+%s:%s", notesRefPattern, remoteNotesRefPattern)
//	err := repo.runGitCommandInline("fetch", remote, fetchRefSpec)
//	if err != nil {
//		return err
//	}
//
//	return repo.mergeRemoteNotes(remote, notesRefPattern)
//}
//
//func getRemoteArchiveRef(remote, archiveRefPattern string) string {
//	relativeArchiveRef := strings.TrimPrefix(archiveRefPattern, "refs/devtools/archives/")
//	return "refs/devtools/remoteArchives/" + remote + "/" + relativeArchiveRef
//}
//
//func (repo *GitRepo) mergeRemoteArchives(remote, archiveRefPattern string) error {
//	remoteRefs, err := repo.runGitCommand("ls-remote", remote, archiveRefPattern)
//	if err != nil {
//		return err
//	}
//	for _, line := range strings.Split(remoteRefs, "\n") {
//		lineParts := strings.Split(line, "\t")
//		if len(lineParts) == 2 {
//			ref := lineParts[1]
//			remoteRef := getRemoteArchiveRef(remote, ref)
//			if err := repo.mergeArchives(ref, remoteRef); err != nil {
//				return err
//			}
//		}
//	}
//	return nil
//}
//
//// PullNotesAndArchive fetches the contents of the notes and archives refs from
//// a remote repo, and merges them with the corresponding local refs.
////
//// For notes refs, we assume that every note can be automatically merged using
//// the 'cat_sort_uniq' strategy (the git-appraise schemas fit that requirement),
//// so we automatically merge the remote notes into the local notes.
////
//// For "archive" refs, they are expected to be used solely for maintaining
//// reachability of commits that are part of the history of any reviews,
//// so we do not maintain any consistency with their tree objects. Instead,
//// we merely ensure that their history graph includes every commit that we
//// intend to keep.
//func (repo *GitRepo) PullNotesAndArchive(remote, notesRefPattern, archiveRefPattern string) error {
//	remoteArchiveRef := getRemoteArchiveRef(remote, archiveRefPattern)
//	archiveFetchRefSpec := fmt.Sprintf("+%s:%s", archiveRefPattern, remoteArchiveRef)
//
//	remoteNotesRefPattern := getRemoteNotesRef(remote, notesRefPattern)
//	notesFetchRefSpec := fmt.Sprintf("+%s:%s", notesRefPattern, remoteNotesRefPattern)
//
//	err := repo.runGitCommandInline("fetch", remote, notesFetchRefSpec, archiveFetchRefSpec)
//	if err != nil {
//		return err
//	}
//
//	if err := repo.mergeRemoteNotes(remote, notesRefPattern); err != nil {
//		return err
//	}
//	if err := repo.mergeRemoteArchives(remote, archiveRefPattern); err != nil {
//		return err
//	}
//	return nil
//}
*/