upload-pack: report "not our ref" to client

Make upload-pack report "not our ref" errors to the client as an "ERR" line.
(If not, the client would be left waiting for a response when the server is
already dead.)

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

upload-pack.c

@@ -822,9 +822,13 @@ static void receive_needs(void)
 			use_include_tag = 1;
 
 		o = parse_object(sha1_buf);
-		if (!o)
+		if (!o) {
+			packet_write_fmt(1,
+					 "ERR upload-pack: not our ref %s",
+					 sha1_to_hex(sha1_buf));
 			die("git upload-pack: not our ref %s",
 			    sha1_to_hex(sha1_buf));
+		}
 		if (!(o->flags & WANTED)) {
 			o->flags |= WANTED;
 			if (!((allow_unadvertised_object_request & ALLOW_ANY_SHA1) == ALLOW_ANY_SHA1