title: What is the git clone --filter option's syntax?
body: <p>The Git 2.17 <a href="https://github.com/git/git/blob/master/Documentation/RelNotes/2.17.0.txt" rel="noreferrer">changelog</a> describes this option:</p>
  
  <blockquote>
    <ul>
    <li>The machinery to clone &amp; fetch, which in turn involves packing and 
    unpacking objects, has been told how to omit certain objects using<br>
    the filtering mechanism introduced by another topic.  It now knows<br>
    to mark the resulting pack as a promisor pack to tolerate missing<br>
    objects, laying foundation for "narrow" clones.</li>
    </ul>
  </blockquote>
  
  <p>Is this flag ready to be used, or is it most likely quite unstable? Does anyone know the right syntax to pass? Whatever flags I pass are rejected as being an invalid filter-spec. For example, these were my attempts to filter by directory:</p>
  
  <pre><code>git clone file://path --depth=1 --filter '--subdirectory-filter Assets' TestRepo
  git clone file://path --depth=1 --filter --subdirectory-filter Assets TestRepo
  git clone file://path --depth=1 --filter Assets TestRepo
  </code></pre>