pack-bitmap: initialize bitmap_writer_init() with packing_data

In order to determine its object order, the pack-bitmap machinery keeps
a 'struct packing_data' corresponding to the pack or pseudo-pack (when
writing a MIDX bitmap) being written.

The to_pack field is provided to the bitmap machinery by callers of
bitmap_writer_build() and assigned to the bitmap_writer struct at that
point.

But a subsequent commit will want to have access to that data earlier on
during commit selection. Prepare for that by adding a 'to_pack' argument
to 'bitmap_writer_init()', and initializing the field during that
function.

Subsequent commits will clean up other functions which take
now-redundant arguments (like nr_objects, which is equivalent to
pdata->objects_nr, or pdata itself).

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/pack-objects.c

@@ -1342,7 +1342,7 @@ static void write_pack_file(void)
 
 			if (write_bitmap_index) {
 				bitmap_writer_init(&bitmap_writer,
-						   the_repository);
+						   the_repository, &to_pack);
 				bitmap_writer_set_checksum(&bitmap_writer, hash);
 				bitmap_writer_build_type_index(&bitmap_writer,
 					&to_pack, written_list, nr_written);