git pr -m uses an undocumented and sometimes broken feature of GitHub

[spacewander]

According to
https://discourse.drone.io/t/github-claims-that-merge-refs-are-undocumented-feature/1100

The /merge refs are created only to check the mergeability. If a branch is already merged, the ref will be deleted. If no one visits the pull request page or uses the API to trigger the mergeability check, the ref will not update.

Although we could manually trigger the ref rebuild via visit the pull request web page, it would be better to find an alternative way. Maybe we could find the branch targeted by the pull request via GitHub API, and do the merge locally?