Merge branch 'tb/midx-write-cleanup'

Merge branch 'tb/midx-write-cleanup'

Code clean-up around writing the .midx files.

* tb/midx-write-cleanup:
  pack-bitmap.c: reimplement `midx_bitmap_filename()` with helper
  midx: replace `get_midx_rev_filename()` with a generic helper
  midx-write.c: support reading an existing MIDX with `packs_to_include`
  midx-write.c: extract `fill_packs_from_midx()`
  midx-write.c: extract `should_include_pack()`
  midx-write.c: pass `start_pack` to `compute_sorted_entries()`
  midx-write.c: reduce argument count for `get_sorted_entries()`
  midx-write.c: tolerate `--preferred-pack` without bitmaps