title: How do you attach a new pull request to an existing issue on github?
body: <p>I'm not sure, but I have a vague memory of creating a github pull request with "Issue 4" or something in the title, and it automatically attached itself to Issue 4 in the project that I was submitting it to. I tried it again recently and it didn't work -- it just created a brand new issue instead. I don't see any options like "Attach to issue" on the new pull request page, nor "Open a new pull request for this issue" on the issue page. Is there any way to do this, to help project owners keep their Issues page clean and avoid duplication?</p>
  
  <p><strong>Edit</strong>: To clarify, I know that creating a pull request always creates a <em>new</em> issue. I would like to instead attach the pull request to an <em>existing</em> issue.</p>