Merge branch 'tb/incremental-midx-part-2'

Incrementally updating multi-pack index files. * tb/incremental-midx-part-2: midx: implement writing incremental MIDX bitmaps pack-bitmap.c: use `ewah_or_iterator` for type bitmap iterators pack-bitmap.c: keep track of each layer's type bitmaps ewah: implement `struct ewah_or_iterator` pack-bitmap.c: apply pseudo-merge commits with incremental MIDXs pack-bitmap.c: compute disk-usage with incremental MIDXs pack-bitmap.c: teach `rev-list --test-bitmap` about incremental MIDXs pack-bitmap.c: support bitmap pack-reuse with incremental MIDXs pack-bitmap.c: teach `show_objects_for_type()` about incremental MIDXs pack-bitmap.c: teach `bitmap_for_commit()` about incremental MIDXs pack-bitmap.c: open and store incremental bitmap layers pack-revindex: prepare for incremental MIDX bitmaps Documentation: describe incremental MIDX bitmaps Documentation: remove a "future work" item from the MIDX docs