Policy-bot requesting reviewers even when existing users are present

[asvoboda]

I've noticed on some internal repos where the following occurs:

• On a repo where request-review is enabled
• In the create PR workflow, select a user manually who should review
• Open the PR
• Wait a sec, and policy-bot will still request another human for review

I believe this is kind of a bug. If the selected user satisfies the review request, we should probably skip requesting additional reviews on other users. If the manually selected user is not in the approval set, then we should continue automatically requesting reviewers.