Github now has a "rebase and merge" button which does exactly what we
have been doing manually so far.
I think we can just use it. Differences compared to our old way are:
- We don't get any opportunity to add the "Reviewed-By" line, but we
can use the "Commit" field instead that is shown by "git log
--format=full" and also by Github.
- Github now says "Merged" instead of "Closed".
- It's less work.
I have closed two PRs with that button: #5079 and #5084.
I'll update our Workflow document, OK?