.arcconfig

tests: demonstrate that hgweb renders "pruned" that minirst.format() returns

Notice at the bottom of the help text there's "windows". It's a section that is
in the original help text, but was pruned (because hgweb didn't ask for it).

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