title: Git submodule update
body: <p>I'm not clear on what the following means (from the <a href="http://git-scm.com/docs/git-submodule" rel="noreferrer">Git submodule update</a> documentation):</p>
  
  <blockquote>
    <p>...will make the submodules HEAD be detached, unless <code>--rebase</code> or <code>--merge</code> is specified...</p>
  </blockquote>
  
  <p>How does <code>--rebase</code>/<code>--merge</code> change things?</p>
  
  <p>My main use case is to have a bunch of central repositories, which I will embed via submodules into other repositories. I would like to be able to improve on these central repositories, either directly in their original location, or from within their embedding repositories (the ones that use them via submodule).</p>
  
  <ul>
  <li>From within these submodules, can I create branches/modifications and use push/pull just like I would in regular repositories, or are there things to be cautious about?</li>
  <li>How would I advance the submodule referenced commit from say (tagged) 1.0 to 1.1 (even though the head of the original repository is already at 2.0), or pick which branch's commit is used at all?</li>
  </ul>