Avoid writing out unreachable objects as loose objects when repacking,
if such loose objects will immediately pruned due to its age anyway.
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
