rhg: add error message for paths outside the repository when cwd != root This mirrors the Python implementation. The relative path handling should probably be refactored into a util, but it it out of scope for this change.

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