git-gui: Exit shortcut in MacOSX repaired

Now, as in all OSX apps, there is only one quit menu entry.
It's automatically in the wish menu and calls ::tk::mac::Quit when used.

Signed-off-by: Soeren Finster <sf@9by6.de>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>