Merge branch 'tb/incremental-midx-part-1'

Incremental updates of multi-pack index files.

* tb/incremental-midx-part-1:
  midx: implement support for writing incremental MIDX chains
  t/t5313-pack-bounds-checks.sh: prepare for sub-directories
  t: retire 'GIT_TEST_MULTI_PACK_INDEX_WRITE_BITMAP'
  midx: implement verification support for incremental MIDXs
  midx: support reading incremental MIDX chains
  midx: teach `midx_fanout_add_midx_fanout()` about incremental MIDXs
  midx: teach `midx_preferred_pack()` about incremental MIDXs
  midx: teach `midx_contains_pack()` about incremental MIDXs
  midx: remove unused `midx_locate_pack()`
  midx: teach `fill_midx_entry()` about incremental MIDXs
  midx: teach `nth_midxed_offset()` about incremental MIDXs
  midx: teach `bsearch_midx()` about incremental MIDXs
  midx: introduce `bsearch_one_midx()`
  midx: teach `nth_bitmapped_pack()` about incremental MIDXs
  midx: teach `nth_midxed_object_oid()` about incremental MIDXs
  midx: teach `prepare_midx_pack()` about incremental MIDXs
  midx: teach `nth_midxed_pack_int_id()` about incremental MIDXs
  midx: add new fields for incremental MIDX chains
  Documentation: describe incremental MIDX format

diff --cc builtin/repack.c
Simple merge

diff --cc midx-write.c
Simple merge

diff --cc object-name.c
Simple merge

diff --cc t/README
Simple merge

diff --cc t/t5319-multi-pack-index.sh
Simple merge