i want that registered user can see DIFF button and everything, and non-registered can show only 'recent changes', so my wish is to have a DISABLEACTION function managed by a sort of ACL.

A simple way to solve this problew is it that follow.

in /incl/template.php, change:
case 'diff':
 html_diff();
    break;

with:

 case 'diff':
    if ($_SERVER['REMOTE_USER'])
    {
        $first = is_numeric($_REQUEST['first']) ? intval($_REQUEST['first']) : 0;
        html_diff($first);
    }
    else
        html_login();
    break;

in this way, if you are a logged-in user, you can display the DIFFERENCE section, with code confront.
if you are not logged, it displays a form for login.