Bzr: add support for "uncommit" command

All "bzr uncommit" options are supported, even --dry-run (allowing
user to check the green light)

Change-Id: Ib97ef1333709ad18be53c658c23c97356e4e4ce0
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>