midx: teach `nth_bitmapped_pack()` about incremental MIDXs

midx: teach `nth_bitmapped_pack()` about incremental MIDXs

In a similar fashion as in previous commits, teach the function
`nth_bitmapped_pack()` about incremental MIDXs by translating the given
`pack_int_id` from the concatenated lexical order to a MIDX-local
lexical position.

When accessing the containing MIDX's array of packs, use the local pack
ID. Likewise, when reading the 'BTMP' chunk, use the MIDX-local offset
when accessing the data within that chunk.

(Note that the both the call to prepare_midx_pack() and the assignment
of bp->pack_int_id both care about the global pack_int_id, so avoid
shadowing the given 'pack_int_id' parameter).

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