Git: Do not crash when closing creator after reverting all files in a commit

Keep the commit editor around and disable it instead of closing it. That stops
Qt Creator from closing the editor when bringing the commit editor to the
foreground to ask whether it should be closed.

Task-number: QTCREATORBUG-12099
Change-Id: I6927424675338f845f504cb6f630f05eaf8ad49f
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>