Merge branch 'refs/heads/t/prune' into next

* refs/heads/t/prune:
  Introduce prune --disk-space.
  Revamp com_prune().
  Factor out find_removable_snapshot().
  prune Simplify rm exit code logic.
  find_oldest_removable_snapshot(): Improve log message.
  prune: Print a message if there is nothing to prune.
  get_snapshot_list(): Add comment about sorting.
  prune: Fail gracefully if pre-rm hook fails.