Issues with tar in drush_provision_drupal_provision_backup

To make it run correctly, I had to replace Append option (-r) by Create option (-c) on line 53:


/**
* Implentation of hook_provision_backup()
*/
function drush_provision_drupal_provision_backup($url) {
$backup_file = drush_get_option('backup_file');
// Adds the site directory into the backup file
drush_log(dt("Adding sites directory to !backup_file", array('!backup_file' => $backup_file)), 'backup');
// $result = provision_shell_exec("tar -C %s -p -r -f %s .", drush_get_option('sites_path') . "/$url", $backup_file);
$result = provision_shell_exec("tar -C %s -p -c -f %s .", drush_get_option('sites_path') . "/$url", $backup_file);

if (!$result && !drush_get_option('force', false)) {
drush_set_error('PROVISION_BACKUP_FAILED', dt("Could not back up sites directory for drupal"));
}
}

Adiran,
I would like to have some feedback on this.
Is it a bug?
Is there a design matter?

Thanks.

Alexandre