title: git: Log for 'master' only goes back to date bug?
body: <p>When I do the following in my working copy:</p>
  
  <pre><code>&gt; git diff 'master@{1 hour ago}'
  </code></pre>
  
  <p>It prints</p>
  
  <pre><code>warning: Log for 'master' only goes back to Thu, 14 Jan 2016 15:37:11 -0500.
  </code></pre>
  
  <p>Yet when I do <code>git log master</code>, it clearly goes back farther:</p>
  
  <pre><code>commit bb9c80fb1c3895a9c11f7a60710497cc8406e71f
  Author: Andy Edwards &lt;andy@example.com&gt;
  Date:   Tue Jan 12 22:44:21 2016 -0600
  ...
  </code></pre>
  
  <p>Am I doing something wrong, or is this a git bug?</p>