repack: mark geometric progression of packs as retained

repack: mark geometric progression of packs as retained

In non-geometric repacks, any packs which repack wishes to delete are
handled via the `existing_packs` struct, which has a mechanism to retain
would-be-deleted packs (e.g., if we happened to write a new pack
identical to one otherwise marked for deletion).

In geometric repacks, repack removes any rewritten packs (alternatively,
any packs which were combined in order to restore a geometric
progression) by enumerating them via `pack_geometry_remove_redundant()`.

Prepare to use the `existing_packs` deletion machinery for geometric
repacks by marking any non-kept packs above the geometric split line as
retained. Do the same for promisor packs, which have their own split
point.

This commit only records which packs the later deletion pass must keep;
it does not change which packs are written or removed.

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