add an action for fast-forward merge

[lsf37]

Or possibly a github-app, but it should be possible to do in a pull_request_target action. Might just be a bit slow, and show up in the status checks even when not running.

Requirements:

• only people with write access to the repo should be able to trigger an actual merge
• should only merge when all required checks have passed
• abort when there is no clean ff merge

Why?

• preserves gpg-signed commits and commit hashes

[kent-mcleod]

Did GitHub used to support fast-forward merging and then remove it?

[lsf37]

There used to be a GitHub app at least that supported it, but it is no longer maintained. I don't remember if GitHub itself had a ff-merge button, but I don't think so.

[kent-mcleod]

Right. I guess we just simulated it with ff-merge on bitbucket which then pushed to github. And I guess GitHub doesn't allow it because they want to record who the committer was.

[lsf37]

Just logging an observation here: manual fast-forward merge on the command line still works fine, even with PR review and test requirements:

git checkout master
git pull
git merge --ff-only <my-pr-branch>
git push

does work fine if you have sufficient rights to merge the pull request and if the required conditions for merging the PR are achieved. This makes it possible for instance to use gpg-signed commits (without GitHub invalidating the signature on rebase merges).

It'd still be nice if there was an UI thing that did that for us.