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/plumbing/storer/shallow.go

@@ -0,0 +1,10 @@
+package storer
+
+import "gopkg.in/src-d/go-git.v4/plumbing"
+
+// ShallowStorer is a storage of references to shallow commits by hash,
+// meaning that these commits have missing parents because of a shallow fetch.
+type ShallowStorer interface {
+	SetShallow([]plumbing.Hash) error
+	Shallow() ([]plumbing.Hash, error)
+}