title: To git checkout without overwriting data
body: <p><strong>How can you <code>git-checkout</code> without overwriting the data?</strong></p>
  
  <p>I run </p>
  
  <pre><code> git checkout master
  </code></pre>
  
  <p>I get</p>
  
  <pre><code>error: Entry 'forms/answer.php' would be overwritten by merge. Cannot merge.
  </code></pre>
  
  <p>This is surprising, since I did not know that Git merges when I <code>git-checkout</code>.
  I have always run after the command separately <code>git merge new-feature</code>. 
  This seems to be apparently unnecessary if Git merges at checkout.</p>