rust-repo: don't use on-disk dirstate parents in v1
This mistake was not causing any problems yet since we were never updating
parents from Rust code. This is about to change, so let's fix it.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}