.arcconfig

unstable: use the `_mutablerevs` function when computing content divergent

This is a useful function to get the revision relevant to these computation,
lets make sure all code use it, so that we can improve that `_mutablerevs`
function in a later changeset.

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