Manually Merging a GitHub Pull Request

So you want to merge a pull request on GitHub, but the magical green 'Merge' button's not there, and instead you're told 'This pull request cannot be automatically merged'.

If you have the pull request branch checked out on your machine as <BRANCH>, and you're on master, here's the basic command that the merge button's doing:

git merge --no-ff --log -m "Merge pull request #<PR_NUMBER> from <USER>/<BRANCH>" <BRANCH>