Refactor how sub VCS clients redefine commands like diff, log, ...
This allows sensible code shrinks in Bazaar and Mercurial plugins

Change-Id: Ifeaf366c5c1c2eee776ce5d6bc748c3872e53f5a
Merge-request: 2201
Reviewed-by: Tobias Hunger <tobias.hunger@nokia.com>
Reviewed-on: http://codereview.qt.nokia.com/3403


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>