.arcconfig

tests: drop extra "file:" prefix from paths in narrow test

It looks like these were added by mistake in f4d4bd8c8911 (narrow: add
a --narrowspec flag to clone command, 2018-08-08).

Differential Revision: https://phab.mercurial-scm.org/D4531

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