.arcconfig

dirstate: only reload the dirstate when it may have changed

This reinstall the equivalent of what the `filecache` was doing. However it does
it at the dirstate level.

There is a double motivation for this:
- This avoid duplicating logic with the dirstate "identity" logic.
- This increase the lifetime of the `dirstate` object, helping to implement
  change scoping.

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