Does not allow upgrading away from metadirs

[Jelmer Vernooij]

  affects bzr
  status confirmed
  importance low

BzrDirMetaFormat1.get_converter() in its current form does not allow
migration away. This makes it impossible to e.g. have a plugin that
provides a new control dir format and support upgrading to that control
dir format.

It seems suboptimal that the format you are upgrading away from is asked
to provide a converter to the target format, since that format is
generally older.