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.
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