Implement git refs API for listing references (branches, tags and other) (#5354)

* Inital routes to git refs api

* Git refs API implementation

* Update swagger

* Fix copyright

* Make swagger happy add basic test

* Fix test

* Fix test again :)

vendor/gopkg.in/src-d/go-git.v4/storage/filesystem/index.go

@@ -0,0 +1,47 @@
+package filesystem
+
+import (
+	"os"
+
+	"gopkg.in/src-d/go-git.v4/plumbing/format/index"
+	"gopkg.in/src-d/go-git.v4/storage/filesystem/dotgit"
+	"gopkg.in/src-d/go-git.v4/utils/ioutil"
+)
+
+type IndexStorage struct {
+	dir *dotgit.DotGit
+}
+
+func (s *IndexStorage) SetIndex(idx *index.Index) (err error) {
+	f, err := s.dir.IndexWriter()
+	if err != nil {
+		return err
+	}
+
+	defer ioutil.CheckClose(f, &err)
+
+	e := index.NewEncoder(f)
+	err = e.Encode(idx)
+	return err
+}
+
+func (s *IndexStorage) Index() (i *index.Index, err error) {
+	idx := &index.Index{
+		Version: 2,
+	}
+
+	f, err := s.dir.Index()
+	if err != nil {
+		if os.IsNotExist(err) {
+			return idx, nil
+		}
+
+		return nil, err
+	}
+
+	defer ioutil.CheckClose(f, &err)
+
+	d := index.NewDecoder(f)
+	err = d.Decode(idx)
+	return idx, err
+}