Git: Save settings

VCSBaseClient handles saving of settings for us, but we do not
use that yet, so we have to do that ourselves:-(

Change-Id: I731d01d429497581fad814effd68bbd784e05fe8
Reviewed-on: http://codereview.qt-project.org/6080


Reviewed-by: Tobias Hunger <tobias.hunger@nokia.com>