Gerrit: Cleanup GerritPlugin

* Use Git::Internal namespace
* Use a static function for accessing GitClient
* Avoid needless data() calls for QSharedPointer

Change-Id: I590b75bbee4831be368894c1844c29237e7800eb
Reviewed-by: André Hartmann <aha_1980@gmx.de>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>