v0.2.11

Changed

• sync_tree's skip check is now rsync's --update rule: sizes
  match and the target's mtime is at least as new as the source's
  (previously: sizes match and mtimes are exactly equal to the
  second). Tolerates copy_file implementations that don't preserve
  mtime — after a fresh copy the target's mtime is "now", still ≥
  source, so the next run skips instead of pointlessly re-copying.
  Known hole: an upstream rollback with identical size and an older
  mtime won't be detected; upstream YTD snapshots in this project
  are monotonic, so this is safe in practice. copy_file
  implementations are no longer required to preserve mtime — the
  explicit contract is now atomicity only.