pack-bitmap: drop filter in prepare_bitmap_walk()

pack-bitmap: drop filter in prepare_bitmap_walk()

Now that all consumers of prepare_bitmap_walk() have populated the
'filter' member of 'struct rev_info', we can drop that extra parameter
from the method and access it directly from the 'struct rev_info'.

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