dirstate-map: add a missing debug wait point when accessing the v2 docket fc8e37c380d3 added synchronization points to the dirstate to allow for race condition testing without actually requiring a time-based race condition to happen. This changes adds the `pre-read-file` wait point before we read the docket, since callers might ask for the parents before anything else is read, leading to the first read being done before the wait point. This removes some differences in test output which were presumed to be speed related, but weren't.