title: git diff gives ambigious argument error
body: <p>I recently moved from SVN to git, and trying to learn my way around git.
  I need to find the files that have changed between 2 branches of my repository. I use the following command to that:</p>
  
  <pre><code>git diff branch_2..branch_1
  </code></pre>
  
  <p>I get the follwing error:</p>
  
  <pre><code>fatal: ambiguous argument 'branch_2..branch_1': 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>git branch gives the following o/p:</p>
  
  <pre><code>git branch -a
  * branch_1
  master/origin
  remotes/origin/HEAD -&gt; origin/master
  remotes/origin/branch_2
  remotes/origin/branch_1
  </code></pre>