midx-write.c: enumerate `pack_int_id` values directly

midx-write.c: enumerate `pack_int_id` values directly

Our `midx-write.c::fill_packs_from_midx()` function currently enumerates
the range [0, m->num_packs), and then shifts its index variable up by
`m->num_packs_in_base` to produce a valid `pack_int_id`.

Instead, directly enumerate the range:

    [m->num_packs_in_base, m->num_packs_in_base + m->num_packs)

, which are the original pack_int_ids themselves as opposed to the
indexes of those packs relative to the MIDX layer they are contained
within.

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