Create a views handler for the item revision 'action' field

From versioncontrol.views.inc:

  $data['versioncontrol_item_revisions']['action'] = array(
    'title' => t('Action'),
    'help' => t('The action performed on this item'),
    'field' => array(
      //TODO: Action field
      'handler' => 'views_handler_field_generic',
    ),
  );

We have no handler for the {versioncontrol_item_revisions}.action, but it's gonna be important for making the final version of our pretty commit logs.