ci: enforce single-commit PRs

[strub]

Summary

• Adds a GitHub Actions workflow that fails any PR whose commit count is not exactly 1.
• Uses github.event.pull_request.commits from the PR event payload, so no checkout or git history fetch is required.
• Triggers on opened, synchronize, and reopened — the only PR events that can change the commit count.

The repository is configured so that the check-commit-count job is a required status check, making this enforcement binding.

[strub]

Work as expected when the PR has two commits.

[fdupress]

Can we have a tag that allows escape after review? I'd rather not have to force people to split PRs that make sense to keep over multiple commits. (But you and I could also bypass requirements and merge those by hand.)

[strub]

Let's see when this happens. We can either decide to force merge on our side or to implement a different mechanism.

It seems that github stacked PR are on their way anyway. This should solve this problem.

[fdupress]

@strub the merge queue is failing to merge this, now repeatedly, "due to no response for status checks". All jobs succeed, but the notification job gets skipped. Did you change something in the merge queue configuration?

[fdupress]

@strub the merge queue is failing to merge this, now repeatedly, "due to no response for status checks". All jobs succeed, but the notification job gets skipped. Did you change something in the merge queue configuration?

Answer: yes. The merge is waiting for the single-commit job to succeed, but that job is not configured to run in the merge queue. I'll fiddle this in.