add PlainOpen variant to find .git in parent dirs

options.go

@@ -420,3 +420,14 @@ func (o *GrepOptions) Validate(w *Worktree) error {
 
 	return nil
 }
+
+// PlainOpenOptions describes how opening a plain repository should be
+// performed.
+type PlainOpenOptions struct {
+	// DetectDotGit defines whether parent directories should be
+	// walked until a .git directory or file is found.
+	DetectDotGit bool
+}
+
+// Validate validates the fields and sets the default values.
+func (o *PlainOpenOptions) Validate() error { return nil }

repository.go

@@ -225,7 +225,14 @@ func PlainInit(path string, isBare bool) (*Repository, error) {
 // repository is bare or a normal one. If the path doesn't contain a valid
 // repository ErrRepositoryNotExists is returned
 func PlainOpen(path string) (*Repository, error) {
-	dot, wt, err := dotGitToOSFilesystems(path)
+	return PlainOpenWithOptions(path, &PlainOpenOptions{})
+}
+
+// PlainOpen opens a git repository from the given path. It detects if the
+// repository is bare or a normal one. If the path doesn't contain a valid
+// repository ErrRepositoryNotExists is returned
+func PlainOpenWithOptions(path string, o *PlainOpenOptions) (*Repository, error) {
+	dot, wt, err := dotGitToOSFilesystems(path, o.DetectDotGit)
 	if err != nil {
 		return nil, err
 	}
@@ -246,14 +253,33 @@ func PlainOpen(path string) (*Repository, error) {
 	return Open(s, wt)
 }
 
-func dotGitToOSFilesystems(path string) (dot, wt billy.Filesystem, err error) {
-	fs := osfs.New(path)
-	fi, err := fs.Stat(".git")
-	if err != nil {
+func dotGitToOSFilesystems(path string, detect bool) (dot, wt billy.Filesystem, err error) {
+	if path, err = filepath.Abs(path); err != nil {
+		return nil, nil, err
+	}
+	var fs billy.Filesystem
+	var fi os.FileInfo
+	for {
+		fs = osfs.New(path)
+		fi, err = fs.Stat(".git")
+		if err == nil {
+			// no error; stop
+			break
+		}
 		if !os.IsNotExist(err) {
+			// unknown error; stop
 			return nil, nil, err
 		}
-
+		if detect {
+			// try its parent as long as we haven't reached
+			// the root dir
+			if dir := filepath.Dir(path); dir != path {
+				path = dir
+				continue
+			}
+		}
+		// not detecting via parent dirs and the dir does not exist;
+		// stop
 		return fs, nil, nil
 	}
 

repository_test.go

@@ -407,6 +407,36 @@ func (s *RepositorySuite) TestPlainOpenNotExists(c *C) {
 	c.Assert(r, IsNil)
 }
 
+func (s *RepositorySuite) TestPlainOpenDetectDotGit(c *C) {
+	dir, err := ioutil.TempDir("", "plain-open")
+	c.Assert(err, IsNil)
+	defer os.RemoveAll(dir)
+
+	subdir := filepath.Join(dir, "a", "b")
+	err = os.MkdirAll(subdir, 0755)
+	c.Assert(err, IsNil)
+
+	r, err := PlainInit(dir, false)
+	c.Assert(err, IsNil)
+	c.Assert(r, NotNil)
+
+	opt := &PlainOpenOptions{DetectDotGit: true}
+	r, err = PlainOpenWithOptions(subdir, opt)
+	c.Assert(err, IsNil)
+	c.Assert(r, NotNil)
+}
+
+func (s *RepositorySuite) TestPlainOpenNotExistsDetectDotGit(c *C) {
+	dir, err := ioutil.TempDir("", "plain-open")
+	c.Assert(err, IsNil)
+	defer os.RemoveAll(dir)
+
+	opt := &PlainOpenOptions{DetectDotGit: true}
+	r, err := PlainOpenWithOptions(dir, opt)
+	c.Assert(err, Equals, ErrRepositoryNotExists)
+	c.Assert(r, IsNil)
+}
+
 func (s *RepositorySuite) TestPlainClone(c *C) {
 	r, err := PlainClone(c.MkDir(), false, &CloneOptions{
 		URL: s.GetBasicLocalRepositoryURL(),