Git: Fix assert

Refresh button states after running a checkout: The operation changes
the currently active branch and thus needs to disable e.g. the remove
action.

Change-Id: Id060ec63dbcf3c3132a832c2801073df5f9e91f1
Reviewed-on: http://codereview.qt.nokia.com/2195


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@nokia.com>