repack: introduce `--write-midx=incremental`

Expose the incremental MIDX repacking mode (implemented in an earlier
commit) via a new --write-midx=incremental option for `git repack`.

Add "incremental" as a recognized argument to the --write-midx
OPT_CALLBACK, mapping it to REPACK_WRITE_MIDX_INCREMENTAL. When this
mode is active and --geometric is in use, set the midx_layer_threshold
on the pack geometry so that only packs in sufficiently large tip layers
are considered for repacking.

Two new configuration options control the compaction behavior:

 - repack.midxSplitFactor (default: 2): the factor used in the
   geometric merging condition for MIDX layers.

 - repack.midxNewLayerThreshold (default: 8): the minimum number of
   packs in the tip MIDX layer before its packs are considered as
   candidates for geometric repacking.

Add tests exercising the new mode across a variety of scenarios
including basic geometric violations, multi-round chain integrity,
branching and merging histories, cross-layer object uniqueness, and
threshold-based compaction.

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

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