gitk: Use <Button-2> for context menus on OSX

gitk: Use <Button-2> for context menus on OSX

An OSX user has reported that gitk's context menus are not usable
under OSX because it doesn't provide a way to generate <Button-3>
events.  Users can generate <Button-2> events with command+click,
so use that for the context menus instead on OSX.

Signed-off-by: Paul Mackerras <paulus@samba.org>