Update to latest HEAD.
    
    - Use new comps. mock objects to re-integrate group removal. BZ 996866.
    - Add "weak" comps. groups, for installed groups.
    - Add msg. to help users deal with RepoError failures. BZ 867389.
    - Give msgs about install/trans. obsoletes a higher priority. BZ 991080.
    - waitForLock() raises YumBaseError. BZ 1001154.