hook_path_update() now receives the original alias

hook_path_update() now receives an 'original' key with the old path alias data, so that modules implementing that hook can react to the change that happened.

This was added to solve a regression caused when converting path_save() to a service, as it is no longer possible to pass along additional information.

Example:

function mymodule_path_update($path) {
  if ($path['alias'] != $path['original']['alias']) {
    db_update('mytable')
      ->fields(array('alias' => $path['alias']))
      ->condition('pid', $path['pid'])
      ->execute();
  }
}