midx: add new fields for incremental MIDX chains

midx: add new fields for incremental MIDX chains

The incremental MIDX chain feature is designed around the idea of
indexing into a concatenated lexicographic ordering of object IDs
present in the MIDX.

When given an object position, the MIDX machinery needs to be able to
locate both (a) which MIDX layer contains the given object, and (b) at
what position *within that MIDX layer* that object appears.

To do this, three new fields are added to the `struct multi_pack_index`:

  - struct multi_pack_index *base_midx;
  - uint32_t num_objects_in_base;
  - uint32_t num_packs_in_base;

These three fields store the pieces of information suggested by their
respective field names. In turn, the `num_objects_in_base` and
`num_packs_in_base` fields are used to crawl backwards along the
`base_midx` pointer to locate the appropriate position for a given
object within the MIDX that contains it.

The following commits will update various parts of the MIDX machinery
(as well as their callers from outside of midx.c and midx-write.c) to be
aware and make use of these fields when performing object lookups.

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