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

builtin/pack-objects.c

@@ -1398,7 +1398,8 @@ static void write_pack_file(void)
 
 			if (write_bitmap_index) {
 				bitmap_writer_init(&bitmap_writer,
-						   the_repository, &to_pack);
+						   the_repository, &to_pack,
+						   NULL);
 				bitmap_writer_set_checksum(&bitmap_writer, hash);
 				bitmap_writer_build_type_index(&bitmap_writer,
 							       written_list);