rust-dirstate-v2: don't write dirstate if data file has changed This fixes the following race: - process A reads the dirstate - process B reads and writes the dirstate - process A writes the dirstate This either resulted in losing what process B had just written or a crash because the `uuid` had changed and we were trying to write to a file that doesn't exist. More explanations inside. This doesn't fix the issue for dirstate-v1, a later patch addresses it.