Merge branch 'tb/prepare-midx-pack-cleanup' into jch

Improvement on Multi-pack-index API.

* tb/prepare-midx-pack-cleanup:
  midx: return a `packed_git` pointer from `prepare_midx_pack()`
  midx-write.c: extract inner loop from fill_packs_from_midx()
  midx-write.c: guard against incremental MIDXs in want_included_pack()
  midx: access pack names through `nth_midxed_pack_name()`

diff --cc midx-write.c
Simple merge

diff --cc midx.c
Simple merge

diff --cc pack-bitmap.c
Simple merge

diff --cc t/helper/test-read-midx.c
Simple merge