Merge branch 'tb/pack-revindex-api'

Abstract accesses to in-core revindex that allows enumerating
objects stored in a packfile in the order they appear in the pack,
in preparation for introducing an on-disk precomputed revindex.

* tb/pack-revindex-api: (21 commits)
  for_each_object_in_pack(): clarify pack vs index ordering
  pack-revindex.c: avoid direct revindex access in 'offset_to_pack_pos()'
  pack-revindex: hide the definition of 'revindex_entry'
  pack-revindex: remove unused 'find_revindex_position()'
  pack-revindex: remove unused 'find_pack_revindex()'
  builtin/gc.c: guess the size of the revindex
  for_each_object_in_pack(): convert to new revindex API
  unpack_entry(): convert to new revindex API
  packed_object_info(): convert to new revindex API
  retry_bad_packed_offset(): convert to new revindex API
  get_delta_base_oid(): convert to new revindex API
  rebuild_existing_bitmaps(): convert to new revindex API
  try_partial_reuse(): convert to new revindex API
  get_size_by_pos(): convert to new revindex API
  show_objects_for_type(): convert to new revindex API
  bitmap_position_packfile(): convert to new revindex API
  check_object(): convert to new revindex API
  write_reused_pack_verbatim(): convert to new revindex API
  write_reused_pack_one(): convert to new revindex API
  write_reuse_object(): convert to new revindex API
  ...

diff --cc builtin/gc.c
Simple merge

diff --cc packfile.c
Simple merge