git-gui: Don't start 'gitk --all' on Mac OS X.

Since gitk is currently broken on Mac OS X and is unable to start itself
when given command line parameters just don't offer the "Visual All
Branches" menu option on Mac OS X.

Once this feature of gitk is fixed we should change this section of code
to make sure a working version of gitk will be executed before we offer
the option up to the user.

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