Merge branch 'jk/repack-keep-unreachable'

"git repack" learned the "--keep-unreachable" option, which sends
loose unreachable objects to a pack instead of leaving them loose.
This helps heuristics based on the number of loose objects
(e.g. "gc --auto").

* jk/repack-keep-unreachable:
repack: extend --keep-unreachable to loose objects
repack: add --keep-unreachable option
repack: document --unpack-unreachable option