Single-commit `[ci skip` PRs don't get merged

[rodrigol-chan]

Just now, we had a situation where a PR would not be merged. The PR had a single commit containing [ci skip], so our CI provider would not run its checks. Due to that, it would seem Hoff got stuck waiting for a hook that would never come. Having a PR like this in a queue will effectively stall the merge trains.

At least two options:

• For single-commit PRs with [ci skip], just merge them. Not ideal, due to the Not Rocket Science principle.
• Always create a merge commit so that CI is run.

[Riscky]

Always create a merge commit so that CI is run

I'm in favor of this option. We're already creating merge commits in most situations, and having the same behavior everywhere makes the code easier to reason about. We also add the PR number to the merge commit title, information that is currently lost in the situation where we don't make a merge commit.