You can see the line numbers in the diff ( @@ -658,6 +658,7 @@ )
In file domain_mapping.php line 658 you should find the line
$url = get_original_url( 'siteurl' );
A bit further down after
$url .= "/wp-login.php";
add the line
$url = apply_filters( 'login_url', $url );
and save. That should do it if you have the same problem.