git-gui: Create a .app file on MacOS X if requested.

git-gui: Create a .app file on MacOS X if requested.

If a user works with a repository frequently they may want to just
create an icon they can use to launch git-gui against that repository.

Since we already support this concept on Windows we can do the same on
Mac OS X by creating a .app file with a tiny shell script in it that
sets up the necessary environment then invokes our script.

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