pack-bitmap: drop repository argument from prepare_midx_bitmap_git()

pack-bitmap: drop repository argument from prepare_midx_bitmap_git()

We never look at the repository argument which is passed. This makes
sense, since the multi_pack_index struct already tells us everything we
need to access the files in its associated object directory.

Signed-off-by: Jeff King <redacted>
Reviewed-by: Taylor Blau <redacted>
Signed-off-by: Junio C Hamano <redacted>