title: Outdated PR diff
body: <p>I've had a situation where a PR diff (from a feature branch to master) is different to a diff generated by comparing that ame feature branch to master.</p>
  
  <p>The only thing I can think of is that GitHub 'freezes' the PR diff at the time of creation, and then that diff gets 'outdated' as master moves on (other feature branches get merged in).</p>
  
  <p>Does GitHub 'freeze' PR diffs? If so, is there any way to 'refresh' it? Or is there a different reason for the diffs to be different?</p>