Be resilient to mistakes that currently cause wpt-pr-bot to ask everyone to review

[zcorpan]

Sometimes people make mistakes with git (myself included) that cause dozens or hundreds of unrelated commits to be included in a pull request. @wpt-pr-bot responds by asking for review from lots of people. This is both embarrassing for the person who made the mistake and is a waste of time for the reviewers.

It would be good if the bot could detect somehow that something bad has happened and avoid asking lots of people to review.

Related issue #35