Merge branch 'ps/repack-avoid-noop-midx-rewrite' into next

Merge branch 'ps/repack-avoid-noop-midx-rewrite' into next

Even when there is no changes in the packfile and no need to
recompute bitmaps, "git repack" recomputed and updated the MIDX
file, which has been corrected.

* ps/repack-avoid-noop-midx-rewrite:
  midx-write: skip rewriting MIDX with `--stdin-packs` unless needed
  midx-write: extract function to test whether MIDX needs updating
  midx: fix `BUG()` when getting preferred pack without a reverse index