Merge Pull Requests by Fast Forwarding the Target Branch

This action merges a pull request by fast forwarding the target branch. The action is triggered when an authorized user adds a comment containing /fast-forward to the pull request.

The ability to fast forward a branch (the equivalent of doing git merge --ff-only) is needed to have an unmodified, linear history. Unfortunately, it is not currently possible to fast forward a branch using GitHub's web UX. Even when it is possible to do a fast forward, GitHub rewrites the commits, which changes their commit hashes, and destroys any signatures. This action improves the situation a bit by making it possible to fast forward directly from the web UX by posting a comment on the pull request.