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
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}