title: Can't checkout master branch
body: <p>I've been working in a branch with some code to improve an application. Now that I this is finished, I want to merge this branch with master branch, but I can't. I first try to checkout master branch like this:</p>
  
  <pre><code>$ git checkout master
  </code></pre>
  
  <p>but I get this error:</p>
  
  <pre><code>error: The following untracked working tree files would be overwritten by checkout:
  vendor/bin/doctrine
  vendor/bin/doctrine.php
  ...
  </code></pre>
  
  <p>but the vendor directory is included in <code>.gitignore</code> file.
  If I do:</p>
  
  <pre><code>$ git status
  </code></pre>
  
  <p>I get this:</p>
  
  <pre><code># On branch mejoras_contralador
  nothing to commit (working directory clean)
  </code></pre>
  
  <p>After this, I would do:</p>
  
  <pre><code>$ git merge branch
  </code></pre>
  
  <p>How to avoid this error?</p>