git-gui: Toggle between new commit and amend commit modes.

git-gui: Toggle between new commit and amend commit modes.

I was starting to find it annoying that once you entered the 'Amend Last'
mode there was no way to go back to the 'New Commit' mode without quitting
and restarting git-gui.  Its just confusing for the end-user.

Now we can flip back and forth between a new commit and an amend commit
through a pair of radio buttons on the header of the commit buffer area
and through a pair of radio menu buttons in the Commit menu.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>