Merge branch 'tb/gc-recent-object-hook'

"git pack-objects" learned to invoke a new hook program that
enumerates extra objects to be used as anchoring points to keep
otherwise unreachable objects in cruft packs.

* tb/gc-recent-object-hook:
  gc: introduce `gc.recentObjectsHook`
  reachable.c: extract `obj_is_recent()`

diff --cc t/t5304-prune.sh
Simple merge