git-gui: Save geometry before the window layout is damaged

git-gui: Save geometry before the window layout is damaged

Because Tk does not assure us the order that it will process
children in before it destroys the main toplevel we cannot safely
save our geometry data during a "bind . <Destroy>" event binding.
The geometry may have already changed as a result of a one or
more children being removed from the layout.  This was pointed
out in gitk by Mark Levedahl, and patched over there by commit
b6047c5a8166a71e01c6b63ebbb67c6894d95114.

So we now also use "wm protocol . WM_DELETE_WINDOW" to detect when
the window is closed by the user, and forward that close event to
our main do_quit routine.

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