exchange: add fast path for subrepo check on push Try to check if .hgsub and .hgsubstate exist at all before looking for them in every changeset to be pushed. The latter can be quite expensive for large repositories and the existance check is almost free. Differential Revision: https://phab.mercurial-scm.org/D11956

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