.arcconfig

templatefilters: raise ProgrammingError if unencodable type passed to json()

This shouldn't happen for any template data types (though I know it does
because of some templater bugs.) Let's clarify it is a bug.

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