pluginlib.php plugin_external_source() functions ignore Mercurial repositories

1. Create a Mercurial repository holding a 3d party plugin in you Moodle installation ensuring the plugin's folder is writeable by the web server (repository folder ".hg" should be just inside you plugin directory, not outside it - e.g. version.php for example should be in the root directory of repository). You would want an obsolete version of plugin with newer version on Moodle plugin database. 2. Try to update this plugin from Moodle Plugins database and see that you get confirmation warning about plugin been from external mercurial source and not been able to update it from repository anymore if you continue. Say "no" to updating, to be able to do next test.