Warn when send-pack does nothing

If you try to push into an empty repository with no ref arguments to
git push, it doesn't do anything and doesn't say anything. This adds a
warning when send-pack isn't going to push anything, so you don't
assume that it silently did what you wanted.
Signed-off-by: NDaniel Barkalow <barkalow@iabervon.org>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

send-pack.c

@@ -190,6 +190,12 @@ static int send_pack(int in, int out, int nr_refspec, char **refspec)
 	if (match_refs(local_refs, remote_refs, &remote_tail,
 		       nr_refspec, refspec, send_all))
 		return -1;
+
+	if (!remote_refs) {
+		fprintf(stderr, "No refs in common and none specified; doing nothing.\n");
+		return 0;
+	}
+
 	/*
 	 * Finally, tell the other end!
 	 */