pack-bitmap.c: support bitmap pack-reuse with incremental MIDXs

pack-bitmap.c: support bitmap pack-reuse with incremental MIDXs

In a similar fashion as previous commits in the first phase of
incremental MIDXs, enumerate not just the packs in the current
incremental MIDX layer, but previous ones as well.

Likewise, in reuse_partial_packfile_from_bitmap(), when reusing only a
single pack from a MIDX, use the oldest layer's preferred pack as it is
likely to contain the largest number of reusable sections.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Acked-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>