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