pack-bitmap-write.c: write lookup table extension

pack-bitmap-write.c: write lookup table extension

The bitmap lookup table extension was documented by an earlier
change, but Git does not yet know how to write that extension.

Teach Git to write bitmap lookup table extension. The table contains
the list of `N` <commit_pos, offset, xor_row>` triplets. These
triplets are sorted according to their commit pos (ascending order).
The meaning of each data in the i'th triplet is given below:

  - commit_pos stores commit position (in the pack-index or midx).
    It is a 4 byte network byte order unsigned integer.

  - offset is the position (in the bitmap file) from which that
    commit's bitmap can be read.

  - xor_row is the position of the triplet in the lookup table
    whose bitmap is used to compress this bitmap, or `0xffffffff`
    if no such bitmap exists.

Mentored-by: Taylor Blau <me@ttaylorr.com>
Co-mentored-by: Kaartic Sivaraam <kaartic.sivaraam@gmail.com>
Co-authored-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Abhradeep Chakraborty <chakrabortyabhradeep79@gmail.com>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>