pack-bitmap: write multi-pack bitmaps

pack-bitmap: write multi-pack bitmaps

Write multi-pack bitmaps in the format described by
Documentation/technical/bitmap-format.txt, inferring their presence with
the absence of '--bitmap'.

To write a multi-pack bitmap, this patch attempts to reuse as much of
the existing machinery from pack-objects as possible. Specifically, the
MIDX code prepares a packing_data struct that pretends as if a single
packfile has been generated containing all of the objects contained
within the MIDX.

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