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  We can also have the case where there are links to a file that are
  not in the tree being transferred.  There's nothing we can do about
  that.  Because we rename the destination into place after writing,
  any hardlinks to the old file are always going to be orphaned.  In
  fact that is almost necessary because otherwise we'd get really
  confused if we were generating checksums for one name of a file and
  modifying another.