.arcconfig

exchange: allow passing no includes/excludes to `pull()`

I would expect that `exchange.pull(includepats=[])` results in an empty list of
include patterns to be passed to the remote, but it doesn't currently because we
check for any truthy value instead of checking specifically for `not None`.

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