Merge branch 'tb/midx-race-in-pack-objects'

The multi-pack-index code did not protect the packfile it is going to depend on from getting removed while in use, which has been corrected. * tb/midx-race-in-pack-objects: builtin/pack-objects.c: ensure pack validity from MIDX bitmap objects builtin/pack-objects.c: ensure included `--stdin-packs` exist builtin/pack-objects.c: avoid redundant NULL check pack-bitmap.c: check preferred pack validity when opening MIDX bitmap