The behavior of repack and pack-objects has been updated to avoid
losing objects to a '.keep' file that appears mid-run. Instead of
pack-objects looking for '.keep' files independently, repack now
hands it a snapshot of kept packs via a new --keep-pack-from-file
option.
* qs/repack-kept-pack-fix:
repack: tell pack-objects which packs are kept
pack-objects: add --keep-pack-from-file
pack-objects: sort --keep-pack list for lookup
pack-objects: reset kept-pack cache for cruft walk
pack-objects: keep --keep-pack open when following