title: Github: Remove commit from pull request
body: <p>Yesterday I submitted my first ever pull request to a project, this one in fact:</p>
  
  <p><a href="https://github.com/dotnet/corefx/pull/484" rel="nofollow">https://github.com/dotnet/corefx/pull/484</a></p>
  
  <p>The feedback I got is that I need to clean it up because I put two commits into it when there should have been one. The first commit in the list was me getting the latest changes from the master and merging them into my fork, it should never have been part of the pull request I made but it seems I accidentally included it.</p>
  
  <p>Given that I have never done one of these before how do I go about modifying it to remove the unnecessary commit? Reading around I'm not sure if I'm meant to rebase my branch, and if so I'm not sure how to continue. </p>