adjust behaviour of rename to match php rather than bash

[michael-donat]

#19 for details

currently renaming a folder over a folder puts source inside destination

this should either completely override destination or fail if destination non empty

[michael-donat]

Digging further,

<?php
        mkdir('/tmp/a');
        mkdir('/tmp/b');

        rename('/tmp/a', '/tmp/b');

Above removed a and replaced with b.

While

<?php
        mkdir('/tmp/aa', 0777, true);
        mkdir('/tmp/bb', 0777, true);
        file_put_contents('/tmp/bb/file', 'data');

        rename('/tmp/aa', '/tmp/bb');

Resulted in rename(/tmp/aa,/tmp/bb): Directory not empty

This is probably the behaviour to be expected. I will adjust as soon as I have some time.