move plumbing from top level package to plumbing (#183)

* plumbing: rename Object -> EncodedObject.

* plumbing/storer: rename ObjectStorer -> EncodedObjectStorer.

* move difftree to plumbing/difftree.

* move diff -> utils/diff

* make Object/Tag/Blob/Tree/Commit/File depend on storer.

* Object and its implementations now depend only on
  storer.EncodedObjectStorer, not git.Repository.
* Tests are decoupled accordingly.

* move Object/Commit/File/Tag/Tree to plumbing/object.

* move Object/Commit/File/Tag/Tree to plumbing/object.
* move checkClose to utils/ioutil.

* move RevListObjects to plumbing/revlist.Objects.

* move DiffTree to plumbing/difftree package.

* rename files with plural nouns to singular

* plumbing/object: add GetBlob/GetCommit/GetTag/GetTree.

1 files changed, 67 insertions, 0 deletions

diff --git a/plumbing/object/commit_walker.go b/plumbing/object/commit_walker.go
new file mode 100644
index 0000000..dcce6b9
--- /dev/null
+++ b/plumbing/object/commit_walker.go
@@ -0,0 +1,67 @@
+package object
+
+import (
+	"io"
+
+	"gopkg.in/src-d/go-git.v4/plumbing"
+)
+
+type commitWalker struct {
+	seen  map[plumbing.Hash]bool
+	stack []*CommitIter
+	start *Commit
+	cb    func(*Commit) error
+}
+
+// WalkCommitHistory walks the commit history
+func WalkCommitHistory(c *Commit, cb func(*Commit) error) error {
+	w := &commitWalker{
+		seen:  make(map[plumbing.Hash]bool),
+		stack: make([]*CommitIter, 0),
+		start: c,
+		cb:    cb,
+	}
+
+	return w.walk()
+}
+
+func (w *commitWalker) walk() error {
+	var commit *Commit
+
+	if w.start != nil {
+		commit = w.start
+		w.start = nil
+	} else {
+		current := len(w.stack) - 1
+		if current < 0 {
+			return nil
+		}
+
+		var err error
+		commit, err = w.stack[current].Next()
+		if err == io.EOF {
+			w.stack = w.stack[:current]
+			return w.walk()
+		}
+
+		if err != nil {
+			return err
+		}
+	}
+
+	// check and update seen
+	if w.seen[commit.Hash] {
+		return w.walk()
+	}
+
+	w.seen[commit.Hash] = true
+	if commit.NumParents() > 0 {
+		w.stack = append(w.stack, commit.Parents())
+	}
+
+	if err := w.cb(commit); err != nil {
+		return err
+	}
+
+	return w.walk()
+}