Git: Remove branches from show header

It is too slow on large repositories. Will be revived in 3.2 with a
trigger button.

Task-number: QTCREATORBUG-11293
Change-Id: I6ba2028b280821b2acb49b7664ec9ece063c8981
Reviewed-by: Przemyslaw Gorszkowski <pgorszkowski@gmail.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>