pack-bitmap: remember pseudo-merge parents

pack-bitmap: remember pseudo-merge parents

write_pseudo_merges() currently builds an array of temporary bitmaps for
the parent set of each pseudo-merge, then serializes those bitmaps later
while writing the extension.

Move those parent bitmaps onto the corresponding bitmapped_commit
entries instead. This keeps the on-disk output unchanged, but gives the
parent bitmap the same lifetime and access pattern that later changes
will use when pseudo-merge object bitmaps are built before the write
step.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>