.arcconfig

branchmap: explicitly convert file into iterator

Follows up 2a4bfbb52111. This is required for httprangereader, which is not
an iterable itself.

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