rhg: signal when falling back in logs
We use the `trace` level for the actual message because it can get really busy.
Differential Revision: https://phab.mercurial-scm.org/D11814
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}