knotserver: better support for empty repos

- setting default-branch now works on repos that have empty default
branches
- RepoIndex now returns branch-list if available on empty repos

knotserver/git/git.go

···
 	branches := []types.Branch{}
 
 	defaultBranch, err := g.FindMainBranch()
-	if err != nil {
-		return nil, fmt.Errorf("getting default branch", "error", err.Error())
-	}
 
 	_ = bi.ForEach(func(ref *plumbing.Reference) error {
 		b := types.Branch{}

knotserver/routes.go

···
 
 	gr, err := git.Open(path, ref)
 	if err != nil {
+		plain, err2 := git.PlainOpen(path)
+		if err2 != nil {
+			l.Error("opening repo", "error", err2.Error())
+			notFound(w)
+			return
+		}
+		branches, _ := plain.Branches()
+
 		log.Println(err)
+
 		if errors.Is(err, plumbing.ErrReferenceNotFound) {
 			resp := types.RepoIndexResponse{
-				IsEmpty: true,
+				IsEmpty:  true,
+				Branches: branches,
 			}
 			writeJSON(w, resp)
 			return
···
 
 func (h *Handle) Branches(w http.ResponseWriter, r *http.Request) {
 	path, _ := securejoin.SecureJoin(h.c.Repo.ScanPath, didPath(r))
-	l := h.l.With("handler", "Branches")
 
-	gr, err := git.Open(path, "")
+	gr, err := git.PlainOpen(path)
 	if err != nil {
 		notFound(w)
 		return
 	}
 
-	branches, err := gr.Branches()
-	if err != nil {
-		l.Error("getting branches", "error", err.Error())
-		writeError(w, err.Error(), http.StatusInternalServerError)
-		return
-	}
+	branches, _ := gr.Branches()
 
 	resp := types.RepoBranchesResponse{
 		Branches: branches,
···
 		return
 	}
 
-	gr, err := git.Open(path, "")
+	gr, err := git.PlainOpen(path)
 	if err != nil {
 		notFound(w)
 		return

types/repo.go

···
 }
 
 type TagReference struct {
-	Reference `json:"ref,omitempty"`
-	Tag       *object.Tag `json:"tag,omitempty"`
-	Message   string      `json:"message,omitempty"`
+	Reference
+	Tag     *object.Tag `json:"tag,omitempty"`
+	Message string      `json:"message,omitempty"`
 }
 
 type Reference struct {
···
 }
 
 type RepoBranchResponse struct {
-	Branch Branch `json:"branch,omitempty"`
+	Branch Branch
 }
 
 type RepoDefaultBranchResponse struct {