repack: implement incremental MIDX repacking

Implement the `write_midx_incremental()` function, which builds and
maintains an incremental MIDX chain as part of the geometric repacking
process.

Unlike the default mode which writes a single flat MIDX, the incremental
mode constructs a compaction plan that determines which MIDX layers to
write, compact, or copy, and then executes each step using `git
multi-pack-index` subcommands with the --no-write-chain-file flag.

The repacking strategy works as follows:

 * Acquire the lock guarding the multi-pack-index-chain.

 * A new MIDX layer is always written containing the newly created
   pack(s). If the tip MIDX layer was rewritten during geometric
   repacking, any surviving packs from that layer are also included.

 * Starting from the new layer, adjacent MIDX layers are merged together
   as long as the accumulated object count exceeds half the object count
   of the next deeper layer (controlled by 'repack.midxSplitFactor').

 * Remaining layers in the chain are evaluated pairwise and either
   compacted or copied as-is, following the same merging condition.

 * Write the contents of the new multi-pack-index chain, atomically move
   it into place, and then release the lock.

 * Delete any now-unused MIDX layers.

After writing the new layer, the strategy is evaluated among the
existing MIDX layers in order from oldest to newest. Each step that
writes a new MIDX layer uses "--no-write-chain-file" to avoid updating
the multi-pack-index-chain file. After all steps are complete, the new
chain file is written and then atomically moved into place.

At present, this functionality is exposed behind a new enum value,
`REPACK_WRITE_MIDX_INCREMENTAL`, but has no external callers. A
subsequent commit will expose this mode via `git repack
--write-midx=incremental`.

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

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