Reset ci bot codereview on precheck start

When the 'bot has failed due to random problems in code unrelated to the change being tested, whoever initiated the Precheck will typically initiate another. The first Precheck's −1 is largely irrelevant.in this case, so the second one might just as well clear it in the process of announcing that it's been initiated. Doing so will have the side-benefit that the summary table in the user's dashboard will then not show the −1 , making the later run's result evident in the dashboard, when it shows up, even if it's again a −1.