midx: implement MIDX compaction

midx: implement MIDX compaction

When managing a MIDX chain with many layers, it is convenient to combine
a sequence of adjacent layers into a single layer to prevent the chain
from growing too long.

While it is conceptually possible to "compact" a sequence of MIDX layers
together by running "git multi-pack-index write --stdin-packs", there
are a few drawbacks that make this less than desirable:

 - Preserving the MIDX chain is impossible, since there is no way to
   write a MIDX layer that contains objects or packs found in an earlier
   MIDX layer already part of the chain. So callers would have to write
   an entirely new (non-incremental) MIDX containing only the compacted
   layers, discarding all other objects/packs from the MIDX.

 - There is (currently) no way to write a MIDX layer outside of the MIDX
   chain to work around the above, such that the MIDX chain could be
   reassembled substituting the compacted layers with the MIDX that was
   written.

 - The `--stdin-packs` command-line option does not allow us to specify
   the order of packs as they appear in the MIDX. Therefore, even if
   there were workarounds for the previous two challenges, any bitmaps
   belonging to layers which come after the compacted layer(s) would no
   longer be valid.

This commit introduces a way to compact a sequence of adjacent MIDX
layers into a single layer while preserving the MIDX chain, as well as
any bitmap(s) in layers which are newer than the compacted ones.

Implementing MIDX compaction does not require a significant number of
changes to how MIDX layers are written. The main changes are as follows:

 - Instead of calling `fill_packs_from_midx()`, we call a new function
   `fill_packs_from_midx_range()`, which walks backwards along the
   portion of the MIDX chain which we are compacting, and adds packs one
   layer a time.

   In order to preserve the pseudo-pack order, the concatenated pack
   order is preserved, with the exception of preferred packs which are
   always added first.

 - After adding entries from the set of packs in the compaction range,
   `compute_sorted_entries()` must adjust the `pack_int_id`'s for all
   objects added in each fanout layer to match their original
   `pack_int_id`'s (as opposed to the index at which each pack appears
   in `ctx.info`).

 - When writing out the new 'multi-pack-index-chain' file, discard any
   layers in the compaction range, replacing them with the newly written
   layer, instead of keeping them and placing the new layer at the end
   of the chain.

This ends up being sufficient to implement MIDX compaction in such a way
that preserves bitmaps corresponding to more recent layers in the MIDX
chain.

The tests for MIDX compaction are so far fairly spartan, since the main
interesting behavior here is ensuring that the right packs/objects are
selected from each layer, and that the pack order is preserved despite
whether or not they are sorted in lexicographic order in the original
MIDX chain.

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