Prepare to implement support for reachability bitmaps for the new
incremental multi-pack index (MIDX) feature over the following commits.
This commit begins by first describing the relevant format and usage
details for incremental MIDX bitmaps.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Acked-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>