Fix confusing git-update-ref error message

When git-update-ref has hit the "Ref %s changed to %s" error, I just stare
at it, left puzzled. This patch attempts to reword that to a more useful
and less confusing error message.
Signed-off-by: NPetr Baudis <pasky@suse.cz>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

update-ref.c

@@ -42,7 +42,7 @@ int main(int argc, char **argv)
 
 	if (oldval) {
 		if (memcmp(currsha1, oldsha1, 20))
-			die("Ref %s changed to %s", refname, sha1_to_hex(currsha1));
+			die("Ref %s is at %s but expected %s", refname, sha1_to_hex(currsha1), sha1_to_hex(oldsha1));
 		/* Nothing to do? */
 		if (!memcmp(oldsha1, sha1, 20))
 			exit(0);