Merge branch 'bw/amend-checkbutton'

Change the amend setting from two radio buttons ("New commit" and "Amend commit") to a single checkbutton. The two radio buttons can never be selected together because they are exactly the opposite of each other, so it makes sense to change it to a single checkbutton. * bw/amend-checkbutton: git-gui: convert new/amend commit radiobutton to checkbutton