pack-objects: use rev.filter when possible

pack-objects: use rev.filter when possible

In builtin/pack-objects.c, we use a 'filter_options' global to populate
the --filter=<X> argument. The previous change created a pointer to a
filter option in 'struct rev_info', so we can use that pointer here as a
start to simplifying some usage of object filters.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>