Git: Refactor stash handling

* Replace RebaseManager with ConflictHandler
* Store StashGuard in GitClient, allow popping after
  a whole process is done (e.g. pull with resolved conflicts)

Change-Id: I85784f32f515ff896c73f35303c7de26f8006f59
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>