pack-bitmap.c: avoid repeated `pack_pos_to_offset()` during reuse

When calling `try_partial_reuse()`, the (sole) caller from the function `reuse_partial_packfile_from_bitmap_1()` has to translate its bit position to a pack position. In the MIDX bitmap case, the caller translates from the bit position, to a position in the MIDX's pseudo-pack order (with `pack_pos_to_midx()`), then get a pack offset (with `nth_midxed_offset()`) before finally working backwards to get the pack position in the source pack by calling `offset_to_pack_pos()`. In the non-MIDX bitmap case, we can use the bit position as the pack position directly (see the comment at the beginning of the `reuse_partial_packfile_from_bitmap_1()` function for why). In either case, the first thing that `try_partial_reuse()` does after being called is determine the offset of the object at the given pack position by calling `pack_pos_to_offset()`. But we already have that information in the MIDX case! Avoid re-computing that information by instead passing it in. In the MIDX case, we already have that information stored. In the non-MIDX case, the call to `pack_pos_to_offset()` moves from the function `try_partial_reuse()` to its caller. In total, we'll save one call to `pack_pos_to_offset()` when processing MIDX bitmaps. (On my machine, there is a slight speed-up on the order of ~2ms, but it is within the margin of error over 10 runs, so I think you'd have to have a truly gigantic repository to confidently measure any significant improvement here). Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>