Implement branch comparing and Compare branches tree

This add a new feature, comparing branches. Implementation is as follows:
- Move everything inside the GitWidget into a QStackedWidget
- Whenever the user wants to compare branches, create a new CompareBranchesView
and switch to it
- After the user is done, the compare view is deleted.

Signed-off-by: Waqar Ahmed <waqar.17a@gmail.com>