merge: disable the whole filesystem access loop if [_realfs] is false
This makes it clearer that [auditeddir] is only relevant for
[_realfs] checkers, and makes the non-realfs checkers more performant.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}