Use the correct binary name for GNU diff on Solaris.
Review Request #5793 — Created May 9, 2014 and submitted
People still use Solaris! On Solaris, GNU diff is called 'gdiff'. Some of our
code (particularly in the perforce client) handled this, but the check to make
sure that we were using GNU diff didn't.
I haven't had access to a Solaris machine in over a decade (eep), so I couldn't
test that part, but I ran the unit tests to make sure I didn't fudge up