plumbing: fix empty uploadpack request error

If we have all what we asked for, finish the session and handle error.

This is equivalent of git "Already up to date." message.

Fixes: go-git#328
Fixes: go-git#638

plumbing/transport/internal/common/common.go

@@ -245,6 +245,12 @@ func (s *session) handleAdvRefDecodeError(err error) error {
 // returned with the packfile content. The reader must be closed after reading.
 func (s *session) UploadPack(ctx context.Context, req *packp.UploadPackRequest) (*packp.UploadPackResponse, error) {
 	if req.IsEmpty() {
+		// XXX: IsEmpty means haves are a subset of wants, in that case we have
+		// everything we asked for. Close the connection and return nil.
+		if err := s.finish(); err != nil {
+			return nil, err
+		}
+		// TODO:(v6) return nil here
 		return nil, transport.ErrEmptyUploadPackRequest
 	}
 

remote.go

@@ -552,6 +552,10 @@ func (r *Remote) fetchPack(ctx context.Context, o *FetchOptions, s transport.Upl
 
 	reader, err := s.UploadPack(ctx, req)
 	if err != nil {
+		if errors.Is(err, transport.ErrEmptyUploadPackRequest) {
+			// XXX: no packfile provided, everything is up-to-date.
+			return nil
+		}
 		return err
 	}