Merge pull request using fast forward only, if possible, moving base branch (target branch) to head branch (source branch). Comment success or failure messages on the pull request issue. The goal is to keep branches equal at the end of the merge.
git checkout target_base && git merge source_head --ff-only
As a more advanced use case, this action can also update status API (with success or failure), which can be used to block pr when status checks are required to pass before merging. You can also have different failure messages commented on the pr issue, based on the state of a staging and a production branch.