git-gui: Improve right click context menu binding on all platforms.

git-gui: Improve right click context menu binding on all platforms.

Apparently <Button-3> doesn't work on my single button PowerBook
mouse under Mac OS X.  I'm guessing this is because Tk is stealing
every event and doesn't realize that Control-Button-1 is actually
supposed to invoke the context menu on this platform.

So now we have a utility procedure is_MacOSX to guess if we are
running on a Mac OS X system, and if so setup Control-Button-1 to
also activate what Button-3 should have.  This does mean that I need
to stay away from using Control-Button-1 as a binding in any other
context.  Of course we should use $M1B for that, which is M1 (aka
Command) on Mac OS X so that shouldn't prove to be a problem.

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