repack-geometry: prepare for incremental MIDX repacking

Teach `pack_geometry_init()` to optionally restrict the set of
repacking candidates to only packs in the tip MIDX layer when a
`midx_layer_threshold` is configured. If the tip layer has fewer packs
than the threshold, those packs are excluded entirely; otherwise only
packs in that layer participate in the geometric repack.

Also track whether any tip-layer packs were included in the rollup
(`midx_tip_rewritten`), which a subsequent commit will use to decide
how to update the MIDX chain after repacking.

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

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