Make "branches expanding" on demand and asynchronous.
After "git show" there is clickable text: "Branches: <Expand>" in
description. If user clicks this text then branches for commit is
triggered and done asynchronously.

Task-number: QTCREATORBUG-11293
Done-with: Przemyslaw Gorszkowski <pgorszkowski@gmail.com>
Change-Id: I772cfef823d3f95e2b3060dfb5973157d81fc11a
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>