git-merge: leave sensible reflog message when used as the first level UI.

It used to throw potentially multi-line log message at reflog.
Just record the heads that were given to be merged at the command
line and the action.

Revert the removal of the check in "git-update-ref -m" I made earlier
which was only a work-around for this.

Signed-off-by: Junio C Hamano <junkio@cox.net>

builtin-update-ref.c

@@ -23,6 +23,8 @@ int cmd_update_ref(int argc, const char **argv, const char *prefix)
 			msg = argv[++i];
 			if (!*msg)
 				die("Refusing to perform update with empty message.");
+			if (strchr(msg, '\n'))
+				die("Refusing to perform update with \\n in message.");
 			continue;
 		}
 		if (!strcmp("-d", argv[i])) {