title: Create a git submodule from a specific repo hash or tag
body: <p>I have a project which depends on Twitter bootstrap 2.x, however, when I add bootstrap as a submodule using the following:</p>
  <pre class="lang-bash prettyprint-override"><code>git submodule add https://github.com/twbs/bootstrap.git
  </code></pre>
  <p>This brings in the latest version of bootstrap.</p>
  <p>I would like to create a submodule for a specific tag, but haven't found a way to do that. Is this possible?</p>