Git: Update branches dialog when repository is changed

and on fetch

Task-number: QTCREATORBUG-9667
Change-Id: Id2e5080502bbc28d5f7900dd0433e468dda56c82
Reviewed-by: Robert Loehning <robert.loehning@digia.com>
Reviewed-by: Petar Perisin <petar.perisin@gmail.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: hjk <hjk121@nokiamail.com>