title: GitHub Pull Request to a new Upstream Branch
body: <p>My workflow: I have a fork (origin) of a remote project (upstream). I have created a branch, topic/TICKET. I want to create a pull request of my origin branch to the upstream project in a same-named, brand new "topic/TICKET" branch. In other words, I don't want my pull request toward the master branch or any existing branch.</p>
  
  <p>When using BitBucket I have this option. When I create my PR, I select my origin branch and then the remote branch selection allows "Create new branch (topic/TICKET)".</p>
  
  <p>In GitHub I don't appear to have this option. I can type in the new branch name; but then the option to continue greys out until I select a valid, existing branch upstream.</p>
  
  <p>The annoying work-around is creating the branches upstream before I perform the PR; but it requires asking someone who can commit upstream -- it seems to me like there must be an easier way to do this. </p>
  
  <p>So, is there any way with GitHub to create a pull request to create a new upstream branch, or is this just a known limitation?</p>