Merge branch 'tb/midx-repack-ignore-cruft-packs'

"git multi-pack-index repack/expire" used to repack unreachable
cruft into a new pack, which have been corrected.

* tb/midx-repack-ignore-cruft-packs:
  midx.c: avoid cruft packs with non-zero `repack --batch-size`
  midx.c: remove unnecessary loop condition
  midx.c: replace `xcalloc()` with `CALLOC_ARRAY()`
  midx.c: avoid cruft packs with `repack --batch-size=0`
  midx.c: prevent `expire` from removing the cruft pack
  Documentation/git-multi-pack-index.txt: clarify expire behavior
  Documentation/git-multi-pack-index.txt: fix typo