.arcconfig

parser: add helper function that constructs parsed tree from template

This function will be used as follows:

  build('only(_, _)', x, y)

See the next patch for details.

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