Correct error message in revert/cherry-pick

We now write to MERGE_MSG, not .msg.  I missed this earlier
when I changed the target we write to.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

builtin-revert.c

@@ -133,7 +133,7 @@ static void add_to_msg(const char *string)
 {
 	int len = strlen(string);
 	if (write_in_full(msg_fd, string, len) < 0)
-		die ("Could not write to .msg");
+		die ("Could not write to MERGE_MSG");
 }
 
 static void add_message_to_msg(const char *message)