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>

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