title: git identify the most recent intersection of two branches
body: <p>Is there a shorthand for identifying the last commit common to two branches?
  For example if I have master and then topic1 splits off from master, and they both continue </p>
  
  <pre>
  <code>master:
  a------b------c------d
          \
  topic1:  r------s------t</code>
  </pre>
  
  <p>Is there a way to identify say <code>b</code>?<br>
  For example if there was such a thing as master#topic1 (same as topic1#master) that meant, "the most recent commit shared by both master and topic."</p>
  
  <p>I would like to be able to:</p>
  
  <pre><code>$ git checkout topic1
  $ git diff master#topic1..topic1</code></pre>
  
  <p>I know that I can do:</p>
  
  <pre><code>git diff master..topic</code></pre>
  
  <p>but I don't care about commits <code>c</code> and <code>d</code>.</p>