unionrepo: dynamically create repository type from base repository This is basically the same thing we just did for bundlerepo except for union repositories. .. api:: ``unionrepo.unionrepository()`` is no longer usable on its own. To instantiate an instance, call ``unionrepo.instance()`` or ``unionrepo.makeunionrepository()``. Differential Revision: https://phab.mercurial-scm.org/D4556