git-gui: Disable tearoff menus on Windows, Mac OS X

git-gui: Disable tearoff menus on Windows, Mac OS X

The Windows and Mac OS X platforms do not generally use the tearoff
menu feature found on traditional X11 based systems.  On Windows the
Tk engine does support the feature, but it really is out of place and
just confuses people who aren't used to working on a UNIX system.  On
Mac OS X its not supported for the root menu bar and its submenus, as
it doesn't fit into the overall platform UI model.

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