git-gui: Support a native Mac OS X application bundle

git-gui: Support a native Mac OS X application bundle

If we are building on Darwin (sometimes known as Mac OS X) and we
find the Mac OS X Tk.framework in the expected location we build
a proper Mac OS X application bundle with icons and info list.  The
git-gui and git-citool commands are modified to be very short shell
scripts that just execute the application bundle, starting Tk with
our own info list and icon set.

Although the Makefile change here is rather large it makes for a
much more pleasant user experience on Mac OS X as git-gui now has
its own icon on the dock, in the standard tk_messageBox dialogs,
and the application name now says "Git Gui" instead of "Wish" in
locations such as the menu bar and the alt-tab window.

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