Merge branch 'tb/incremental-midx-part-3.1'

Merge branch 'tb/incremental-midx-part-3.1'

Clean-up "git repack" machinery to prepare for incremental update
of midx files.

* tb/incremental-midx-part-3.1: (49 commits)
  builtin/repack.c: clean up unused `#include`s
  repack: move `write_cruft_pack()` out of the builtin
  repack: move `write_filtered_pack()` out of the builtin
  repack: move `pack_kept_objects` to `struct pack_objects_args`
  repack: move `finish_pack_objects_cmd()` out of the builtin
  builtin/repack.c: pass `write_pack_opts` to `finish_pack_objects_cmd()`
  repack: extract `write_pack_opts_is_local()`
  repack: move `find_pack_prefix()` out of the builtin
  builtin/repack.c: use `write_pack_opts` within `write_cruft_pack()`
  builtin/repack.c: introduce `struct write_pack_opts`
  repack: 'write_midx_included_packs' API from the builtin
  builtin/repack.c: inline packs within `write_midx_included_packs()`
  builtin/repack.c: pass `repack_write_midx_opts` to `midx_included_packs`
  builtin/repack.c: inline `remove_redundant_bitmaps()`
  builtin/repack.c: reorder `remove_redundant_bitmaps()`
  repack: keep track of MIDX pack names using existing_packs
  builtin/repack.c: use a string_list for 'midx_pack_names'
  builtin/repack.c: extract opts struct for 'write_midx_included_packs()'
  builtin/repack.c: remove ref snapshotting from builtin
  repack: remove pack_geometry API from the builtin
  ...