diff: pass the whole diff_options to diffcore_apply_filter()

diff: pass the whole diff_options to diffcore_apply_filter()

The --diff-filter=<arg> option given by the user is kept as a
string, and passed to the underlying diffcore_apply_filter()
function as a string for each resulting path we run number of
strchr() to see if each class of change among ACDMRTXUB is meant to
be given.

Change the function signature to pass the whole diff_options, so
that we can pre-parse this string in the next patch.

Signed-off-by: Junio C Hamano <gitster@pobox.com>