repack: teach MIDX retention about geometric rollups

repack: teach MIDX retention about geometric rollups

When writing an incremental MIDX, existing_packs_retain_midx_packs()
marks packs in the existing MIDX chain as retained. This keeps them from
being deleted by the later existing_packs deletion pass, since retained
MIDX layers may still refer to those packs.

Geometric repacks need a narrower rule. Packs below the split are rolled
up into the newly-written pack, and should remain eligible for deletion
even if the old MIDX chain mentions them. Packs above the split were
marked as retained by the previous commit.

Teach existing_packs_retain_midx_packs() to skip packs which are part of
the geometric rollup. This does not change the current caller's behavior,
since geometric repacks do not yet use the existing_packs deletion path.

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