midx-write: revert use of --stdin-packs

midx-write: revert use of --stdin-packs

This reverts b7d6f23a171 (midx-write.c: use `--stdin-packs` when
repacking, 2024-04-01) and then marks the test created in the previous
change as passing.

The fundamental issue with the reverted change is that the focus on
pack-files separates the object selection from how the multi-pack-index
selects a single pack-file for an object ID with multiple copies among
the tracked pack-files.

The change was made with the intention of improving delta compression in
the resulting pack-file, but that can be resolved with the existing
object list mechanism. There are other potential pitfalls of doing an
object walk at this time if the repository is a blobless partial clone,
and that will require additional testing on top of the one that changes
here.

Signed-off-by: Derrick Stolee <stolee@gmail.com>
Acked-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>