.arcconfig

config: add a .hg/hgrc-not-shared which won't be shared in share-safe mode

Previous patches add a safe mode for sharing repositories which involve sharing
of source requirements and config files.

In certain situations we might need to add a config to source repository which
we does not want to share. For this, we add a `.hg/hgrc-not-shared` which won't
be shared.

This also adds a `--non-shared` flag to `hg config` command to see the
non-shared config.

Differential Revision: https://phab.mercurial-scm.org/D8673

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