.arcconfig

templater: add experimental support for extdata

This is minimal and non-controversial implementation of extdata() template
function. Originally extdata sources were exposed to the keyword namespace,
but I've changed it to a plain function for simplicity.

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