send PACK only if non-delete command present

According to:
https://github.com/git/git/blob/master/Documentation/technical/pack-protocol.txt

> The packfile MUST NOT be sent if the only command used is 'delete'.

Signed-off-by: Stanislav Seletskiy <s.seletskiy@gmail.com>

Author: seletskiy

remote.go

@@ -168,7 +168,17 @@ func (r *Remote) PushContext(ctx context.Context, o *PushOptions) (err error) {
 		}
 	}
 
-	rs, err := pushHashes(ctx, s, r.s, req, hashesToPush, r.useRefDeltas(ar))
+	if len(hashesToPush) == 0 {
+		allDelete = true
+		for _, command := range req.Commands {
+			if command.Action() != packp.Delete {
+				allDelete = false
+				break
+			}
+		}
+	}
+
+	rs, err := pushHashes(ctx, s, r.s, req, hashesToPush, r.useRefDeltas(ar), allDelete)
 	if err != nil {
 		return err
 	}
@@ -1033,10 +1043,11 @@ func pushHashes(
 	req *packp.ReferenceUpdateRequest,
 	hs []plumbing.Hash,
 	useRefDeltas bool,
+	allDelete bool,
 ) (*packp.ReportStatus, error) {
 
 	rd, wr := io.Pipe()
-	req.Packfile = rd
+
 	config, err := s.Config()
 	if err != nil {
 		return nil, err
@@ -1047,15 +1058,20 @@ func pushHashes(
 	// to the channel.
 	done := make(chan error, 1)
 
-	go func() {
-		e := packfile.NewEncoder(wr, s, useRefDeltas)
-		if _, err := e.Encode(hs, config.Pack.Window); err != nil {
-			done <- wr.CloseWithError(err)
-			return
-		}
+	if !allDelete {
+		req.Packfile = rd
+		go func() {
+			e := packfile.NewEncoder(wr, s, useRefDeltas)
+			if _, err := e.Encode(hs, config.Pack.Window); err != nil {
+				done <- wr.CloseWithError(err)
+				return
+			}
 
-		done <- wr.Close()
-	}()
+			done <- wr.Close()
+		}()
+	} else {
+		close(done)
+	}
 
 	rs, err := sess.ReceivePack(ctx, req)
 	if err != nil {