test more combinations of ignore-whitespace options to diff

There are three flags involved (-w -b and --ignore-space-at-eol) which
makes 8 combinations possible in total, but only 3 cases are tested (none,
-w alone and -b alone).

This adds the other 5 cases.

Signed-off-by: Keith Cascio <keith@cs.ucla.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>