title: git diff between HEAD and last commit gives Warning: ignoring dangling symref
body: <p>I am getting some weird output on trying to do diff between HEAD and last commit</p>
  
  <pre><code>Fatal: ambiguous argument 'head^': unknown revision or path not in the working tree.
  use '--' to separate paths from revisions, like this:
  'git &lt;command&gt; [&lt;revision&gt;...] -- [&lt;file&gt;...]'
  </code></pre>
  
  <p>While using <code>git diff &lt;sha&gt; HEAD^</code>, I get the error</p>
  
  <pre><code>warning: ignoring dangling symref head. warning: ignoring dangling symref head.
  fatal: ambiguous argument 'head^': unknown revision or path not in the working tree.
  Use '--' to separate paths from revisions, like this:
  'git &lt;command&gt; [&lt;revision&gt;...] -- [&lt;file&gt;...]'
  </code></pre>
  
  <p>am I missing something?</p>