git-gui: Use a label instead of a button for the back button
Apparently Tk on Mac OS X won't draw a button with an image using a transparent background. Instead it draws the button using some sort of 3D effect, even though I asked for no relief and no border. The background is also not our orange that we expected it to be.
Earlier I had tried this same trick on Windows and it draws the same way as the button did, so I'm going to switch to the label as that seems to be more portable.
Signed-off-by: Shawn O. Pearce <email@example.com>