Add support for +refs

Change-Id: I828efe243c12c66c134a1f12f69499fa96860220

gitiles/client.go

diff --git a/gitiles/client.go b/gitiles/client.go
index 7fec489..9ecf39b 100644
--- a/gitiles/client.go
+++ b/gitiles/client.go
@@ -365,3 +365,22 @@
 
 	panic("unreachable.")
 }
+
+// Refs returns the refs of a repository, optionally filtered by prefix.
+func (s *RepoService) Refs(prefix string) (map[string]*RefData, error) {
+
+	jsonURL := s.service.addr
+	jsonURL.Path = path.Join(jsonURL.Path, s.Name, "+refs")
+	if prefix != "" {
+		jsonURL.Path = path.Join(jsonURL.Path, prefix)
+	}
+	jsonURL.RawQuery = "format=JSON"
+
+	result := map[string]*RefData{}
+	err := s.service.getJSON(&jsonURL, &result)
+	if err != nil {
+		return nil, err
+	}
+
+	return result, err
+}

gitiles/prod_test.go

diff --git a/gitiles/prod_test.go b/gitiles/prod_test.go
index c4fc68a..43ccbef 100644
--- a/gitiles/prod_test.go
+++ b/gitiles/prod_test.go
@@ -81,3 +81,25 @@
 		t.Fatalf("got %v, want %v", got, want)
 	}
 }
+
+func TestProductionRefs(t *testing.T) {
+	gs, err := NewService(Options{
+		Address: "https://gerrit.googlesource.com",
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	repo := gs.NewRepoService("gitiles")
+	if err != nil {
+		t.Fatal(err)
+	}
+	got, err := repo.Refs("refs/heads")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if got["master"] == nil {
+		t.Errorf("got %v, want key 'master'", got)
+	}
+}

gitiles/types.go

diff --git a/gitiles/types.go b/gitiles/types.go
index 56a3da5..ab9721d 100644
--- a/gitiles/types.go
+++ b/gitiles/types.go
@@ -117,3 +117,15 @@
 	return buf.String()
 
 }
+
+// A git reference
+type RefData struct {
+	// The value to which a reference points.
+	Value string
+
+	// If the value points to a tag, the commit that the tag points to.
+	Peeled string
+
+	// If the ref is symbolic, eg. HEAD, the ref to which it points.
+	Target string
+}