Add job forbid-merge-commits to .github/workflows/pull-request.yml.

[motlin]

We recently had a "backwards" merge commit land on the master branch.

It's visible in the git history as the white arrow here.

I wrote and open sourced a GitHub Action to prevent this sort of thing. Its README has more details about why this is a problem and how folks get into this situation. When it happens, the error message is pretty helpful about how to fix up git history.