.arcconfig

subrepo: avoid false unsafe path detection on Windows

Subrepo paths are not normalized for the OS, so what was happening in the
subsequent root path check was:

    root                  -> $TESTTMP\issue1852a\sub/repo
    util.expandpath(...)  -> $TESTTMP\issue1852a\sub/repo
    os.path.realpath(...) -> $TESTTMP\issue1852a\sub\repo

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