dirstate: use `dirstate.change_files` to scope the change in `copy`
This is the way, unless we are not actually touching the working copy.
In such cases we don't need to do something.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}