get_pr currently logs an error if it can't find a merged pull request. It doesn't influence the process's return code, but it is a bit confusing (since it's completely expected for the dry run). How about relocating the message until after we've exited due to the dry run, just before we exit with a non-zero code?

get_pr might call logger.error or logger.warning with different messages, and returning None doesn't always involve a message, so trying to suppress the messages would get a bit unsightly. How about just not call get_pr at all in dry run mode and leaving it untested like the later bits?

@jugglinmike the wpt commit history is far from tidy, but when squashing a commit as in ca1a7a2 feel free to edit the commit message quite liberally and delete parts like the double subject and intermediate steps like "Update manifest_build.py".

If you haven't already noticed and been amused, try git log --grep 'fixup!' for a long list of amusing failures to clean up commit messages :)