Git: Show tracking branch on Branches dialog

Simplify branch model parsing a bit

Change-Id: Id9e41c6c2769397d6eee3ab74de4afbb94111e25
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Petar Perisin <petar.perisin@gmail.com>