Support `.git/commondir` repository layout

Git creates `.git/commondir` when there are custom worktrees (see "git worktree add" related commands).

`.git/commondir` in such case contains a link to another dot-git repository tree, which could contain some folders like:
 - objects;
 - config;
 - refs;
 - etc.

In this PR a new dotgit.RepositoryFilesystem struct is defined, which is billy.Filesystem interface compatible object-wrapper, that can handle commondir and dispatch all operations to the correct file path.

`git.PlainOpen` remain unchanged, but `git.PlainOpenWithOptions` has a new option: `PlainOpenOptions.EnableDotGitCommonDir=true|false` (which is false by default). When `EnableDotGitCommonDir=true` repository-open procedure will read `.git/commondir` (if it exists) and then create dotgit.RepositoryFilesystem object initialized with 2 filesystems. This object then passed into storage and then into dotgit.DotGit as `billy.Filesystem` interface. This object will catch all filesystem operations and dispatch to the correct repository-filesystem (dot-git or common-dot-git) according to the rules described in the doc: https://git-scm.com/docs/gitrepository-layout#Documentation/gitrepository-layout.txt. EnableDotGitCommonDir option will only work with the filesystem-backed storage.

Also worktree_test.go has been adopted from an older, already existing existing PR: https://github.com/src-d/go-git/pull/1098. This PR needs new fixtures added in the following PR: https://github.com/go-git/go-git-fixtures/pull/1.

1 files changed, 75 insertions, 0 deletions

diff --git a/worktree_test.go b/worktree_test.go
index 72bcbd9..c808ebd 100644
--- a/worktree_test.go
+++ b/worktree_test.go
@@ -2052,3 +2052,78 @@ func (s *WorktreeSuite) TestAddAndCommit(c *C) {
 	})
 	c.Assert(err, IsNil)
 }
+
+func (s *WorktreeSuite) TestLinkedWorktree(c *C) {
+	fs := fixtures.ByTag("linked-worktree").One().Worktree()
+
+	// Open main repo.
+	{
+		fs, err := fs.Chroot("main")
+		c.Assert(err, IsNil)
+		repo, err := PlainOpenWithOptions(fs.Root(), &PlainOpenOptions{EnableDotGitCommonDir: true})
+		c.Assert(err, IsNil)
+
+		wt, err := repo.Worktree()
+		c.Assert(err, IsNil)
+
+		status, err := wt.Status()
+		c.Assert(err, IsNil)
+		c.Assert(len(status), Equals, 2) // 2 files
+
+		head, err := repo.Head()
+		c.Assert(err, IsNil)
+		c.Assert(string(head.Name()), Equals, "refs/heads/master")
+	}
+
+	// Open linked-worktree #1.
+	{
+		fs, err := fs.Chroot("linked-worktree-1")
+		c.Assert(err, IsNil)
+		repo, err := PlainOpenWithOptions(fs.Root(), &PlainOpenOptions{EnableDotGitCommonDir: true})
+		c.Assert(err, IsNil)
+
+		wt, err := repo.Worktree()
+		c.Assert(err, IsNil)
+
+		status, err := wt.Status()
+		c.Assert(err, IsNil)
+		c.Assert(len(status), Equals, 3) // 3 files
+
+		_, ok := status["linked-worktree-1-unique-file.txt"]
+		c.Assert(ok, Equals, true)
+
+		head, err := repo.Head()
+		c.Assert(err, IsNil)
+		c.Assert(string(head.Name()), Equals, "refs/heads/linked-worktree-1")
+	}
+
+	// Open linked-worktree #2.
+	{
+		fs, err := fs.Chroot("linked-worktree-2")
+		c.Assert(err, IsNil)
+		repo, err := PlainOpenWithOptions(fs.Root(), &PlainOpenOptions{EnableDotGitCommonDir: true})
+		c.Assert(err, IsNil)
+
+		wt, err := repo.Worktree()
+		c.Assert(err, IsNil)
+
+		status, err := wt.Status()
+		c.Assert(err, IsNil)
+		c.Assert(len(status), Equals, 3) // 3 files
+
+		_, ok := status["linked-worktree-2-unique-file.txt"]
+		c.Assert(ok, Equals, true)
+
+		head, err := repo.Head()
+		c.Assert(err, IsNil)
+		c.Assert(string(head.Name()), Equals, "refs/heads/branch-with-different-name")
+	}
+
+	// Open linked-worktree #2.
+	{
+		fs, err := fs.Chroot("linked-worktree-invalid-commondir")
+		c.Assert(err, IsNil)
+		_, err = PlainOpenWithOptions(fs.Root(), &PlainOpenOptions{EnableDotGitCommonDir: true})
+		c.Assert(err, Equals, ErrRepositoryIncomplete)
+	}
+}