Silent diff in EBox::Types::File::isEqualTo

Remove this unpleasant error:

2013/04/25 01:39:08 ERROR> Sudo.pm:234 EBox::Sudo::_rootError - root command diff -q /var/lib/zentyal/files/squid/archives/Zentyal_security_updates /var/lib/zentyal/tmp/fileList_path failed.