title: what happens to open pull request on a merged branch
body: <p>I have a branch A from master. I created another branch on branch A, let's say branch AA and made few changes on AA.</p>
  
  <p>I then created a pull request of the changes on AA and pointed it to branch A.</p>
  
  <p>Due to some reason, I had to merge branch A back into the master branch.</p>
  
  <p>Now I have an open pull request of AA pointing to A and github shows the Merge Pull Request button to be green (implying that I can safely merge).</p>
  
  <p>Is it fine if I merge the changes I made on AA to branch A? Or should I close the pull request and create a new pull request pointing to the master, since branch A has already been merged into the master?</p>