Non-revisioned entities appear in Diff settings

The goal is to make diff to support any entity, so by default this shows everything. Via direct function calls, you can compare nodes and users if you wanted.

However, I guess that we could use entity info alter to register the entity to the Diff module, and then restrict these administration pages to only those entities that have this flag:

/**
 * Implements hook_entity_info_alter().
 */
function diff_entity_info_alter(&$entity_info) {
  $entity_info['node']['comparable'] = TRUE;
}

Thoughts?