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>