Merge branch 'tb/midx-bitmap-corruption-fix'

Merge branch 'tb/midx-bitmap-corruption-fix'

A bug that made multi-pack bitmap and the object order out-of-sync,
making the .midx data corrupt, has been fixed.

* tb/midx-bitmap-corruption-fix:
  pack-bitmap.c: gracefully fallback after opening pack/MIDX
  midx: read `RIDX` chunk when present
  t/lib-bitmap.sh: parameterize tests over reverse index source
  t5326: move tests to t/lib-bitmap.sh
  t5326: extract `test_rev_exists`
  t5326: drop unnecessary setup
  pack-revindex.c: instrument loading on-disk reverse index
  midx.c: make changing the preferred pack safe
  t5326: demonstrate bitmap corruption after permutation