No diff -b/-w output for all-whitespace changes

Change git-diff's whitespace-ignoring modes to generate
output only if a non-empty patch results, which git-apply
rejects.

Update the tests to look for the new behavior.

Signed-off-by: Greg Bacon <gbacon@dbresearch.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>