> The situation is worse than just the old `be merge` effects, because
> the existence, children, and parents of a particular UUID may be
> revision dependent.  A UUID will always refer to the same
> bugdir/bug/comment, but that bugdir/bug/comment may have different
> relatives.

I'm not sure how to support .children(revision) in the Arch backend
or the older versions of Darcs without checking out a pristine tree
for the revision in question.  That's how we used to support
  BugDir.duplicate_bugdir()
but it doesn't fit well with the new Storage system.  Since I don't
feel strongly about tla or old Darcs support, I'm leaving that
functionality unimplemented.