Add ability to reword and squash commits directly from the history view.

This adds two new menu items, "Reword" and "Squash", to the context menu of the history view. "Reword" prompts for a new commit message for the selected commit. "Squash" can be used to squash multiple (contiguous) selected commits into one, prompting for a commit message as well.

Change-Id: I0b97dc67c3f2b606b35e9dab4300c15165d048c5
Signed-off-by: Maik Schreiber <blizzy@blizzy.de>