Merge branch 'jk/repack-no-explode-objects-from-old-pack' into maint

"git repack" used to write out unreachable objects as loose objects
when repacking, even if such loose objects will immediately pruned
due to its age.

By Jeff King
* jk/repack-no-explode-objects-from-old-pack:
  gc: use argv-array for sub-commands
  argv-array: add a new "pushl" method
  argv-array: refactor empty_argv initialization
  gc: do not explode objects which will be immediately pruned