Fix the new username API

Hi,
you've errors when invoking the 2 new API funcs. Sorry for not creating patches here, but my versioncontrol.module is slightly modded with other stuff i'll file an issue or two for later. This means search&replace game for you :P

return _versioncontrol_call_backend($repository['vcs'], 'account_username_suggestion', $repository, $user);
should be
return _versioncontrol_call_backend($repository['vcs'], 'account_username_suggestion', array($repository, $user));
and
$function = $repository['vcs'] .'is_account_username_valid';
should be
$function = 'versioncontrol_' . $repository['vcs'] .'_is_account_username_valid';