Merge branch 'tb/incremental-midx-part-3.3'

Merge branch 'tb/incremental-midx-part-3.3'

The repacking code has been refactored and compaction of MIDX layers
have been implemented, and incremental strategy that does not require
all-into-one repacking has been introduced.

* tb/incremental-midx-part-3.3:
  repack: allow `--write-midx=incremental` without `--geometric`
  repack: introduce `--write-midx=incremental`
  repack: implement incremental MIDX repacking
  packfile: ensure `close_pack_revindex()` frees in-memory revindex
  builtin/repack.c: convert `--write-midx` to an `OPT_CALLBACK`
  repack-geometry: prepare for incremental MIDX repacking
  repack-midx: extract `repack_fill_midx_stdin_packs()`
  repack-midx: factor out `repack_prepare_midx_command()`
  midx: expose `midx_layer_contains_pack()`
  repack: track the ODB source via existing_packs
  midx: support custom `--base` for incremental MIDX writes
  midx: introduce `--no-write-chain-file` for incremental MIDX writes
  midx: use `strvec` for `keep_hashes`
  midx: build `keep_hashes` array in order
  midx: use `strset` for retained MIDX files
  midx-write: handle noop writes when converting incremental chains