title: Why are git submodules incompatible with svn externals?
body: <p>There are lots of webpages out there suggesting <a href="http://fredericiana.com/2010/01/12/using-svn-repositories-as-git-submodules/" rel="nofollow noreferrer">hackish ways to make svn externals look like git submodules</a>. I have read <a href="https://stackoverflow.com/questions/571232/svn-external-in-git">some accounts</a> of what the difference is, but this doesn't seem very fundamental:</p>
  
  <blockquote>
    <p>Git submodules link to a particular commit in another project's repository, while svn:externals always fetch the latest revision.</p>
  </blockquote>
  
  <p><strong>Why does this difference make them so fundamentally incompatible?</strong> Isn't there a reasonable default we can assume, such as that most svn:externals point to tags that never move?</p>