title: git/github create reverse patch from merged pull request
body: <p>Situation: We have a couple of github pull requests that have been merged that have code that needs to be rolled back.  Interspersed with these merges are others that need to be preserved.  I know we can roll back to prior to the first bad merge and cherry pick changes after that, but I'm hoping we can avoid that hassle.  Is it possible to create some kind of reverse patch based on the two bad pull requests and apply those patches to a new branch, so we can encapsulate the bad changes and restore them when the problems with them have been fixed?  </p>