hacks-guide-minimal-mistakes-theme/_includes/masthead.html

<div class="masthead">
  <div class="site-name">
    <a href="{{ base_path }}/">{{ site.title }}</a>
  </div><!-- /.site-name -->
  <div class="masthead__menu">
    <nav id="site-nav">
      <ul>
        {% for link in site.data.navigation %}
          {% if link.url contains 'http' %}
            {% assign domain = '' %}
            {% else %}
            {% assign domain = base_path %}
          {% endif %}
          <li class="masthead__menu-item"><a href="{{ domain }}{{ link.url }}" {% if link.url contains 'http' %}target="_blank"{% endif %}>{{ link.title }}</a></li>
        {% endfor %}
      </ul>
    </nav>
  </div><!-- /.top-navigation -->
</div><!-- /.navigation-wrapper -->