git-gui: Use a label instead of a button for the back button

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 <spearce@spearce.org>