title: Undo a merge by pull request?
body: <p>Someone accepted a pull request which they shouldn't have. Now we have a bunch of broken code merged in. How do you undo a pull request? I was just going to revert the changes to the commit just before the merge, but I noticed that it merged in a bunch of commits. So now there are all these commits from this person from days before the merge. How do you undo this?</p>