title: Github Comparison View for 2 branches is incorrect?
body: <p>When I do a <a href="https://help.github.com/articles/comparing-commits-across-time/" rel="noreferrer">Github comparison view</a> between <code>master</code> and another branch <code>A</code>, Github seems to be comparing the HEAD version of <code>A</code> with an older, non head version of <code>master</code>.</p>
  
  <p>I looked into it, and from what I can tell it sounds like Github is comparing branch <code>A</code> against a common ancestor of <code>master</code>. It's not actually comparing it to what's currently at the <code>HEAD</code> of master.</p>
  
  <p><strong>Is there a way to diff the HEAD of master against the HEAD of Branch <code>A</code> in Github?</strong></p>
  
  <p>If not, why not? This seems like a feature every developer would want to be able to do. Or is there some process that should be done that I'm perhaps missing? I'd like to be able to create a Pull Request directly from one of these diffs.</p>
  
  <p><strong>EDIT:</strong> Here's an example repo that shows the problem</p>
  
  <p><a href="https://github.com/bradparks/test_github_diff_view" rel="noreferrer">https://github.com/bradparks/test_github_diff_view</a></p>
  
  <ul>
  <li>I created a new repo that contains a single file, README.md</li>
  <li>I set this file to contain a single line <code>v100</code>.</li>
  <li>I then branched master into a new branch <code>A</code>, and changed the line to `v200'</li>
  <li><p>I then changed the value in the master branch to 'v300', and  then compared the 2 branches using the compare link</p>
  
  <p><a href="https://github.com/bradparks/test_github_diff_view/compare/A" rel="noreferrer">https://github.com/bradparks/test_github_diff_view/compare/A</a></p>
  
  <p>and I see the following unexpected result. Why isn't it diffing against <code>v300</code> instead of <code>v100</code>?</p>
  
  <p><a href="https://i.stack.imgur.com/bnaxT.png" rel="noreferrer"><img src="https://i.stack.imgur.com/bnaxT.png" alt="enter image description here"></a></p></li>
  </ul>