title: Add subdirectory of remote repo with git-subtree
body: <p>Is there a way to add a subdirectory of a remote repository into a subdirectory of my repository with git-subtree?</p>
  
  <p>Suppose I have this <strong>main</strong> repository:</p>
  
  <pre><code>/
      dir1
      dir2
  </code></pre>
  
  <p>And this <strong>library</strong> repository:</p>
  
  <pre><code>/
      libdir
          some-file
      some-file-to-be-ignored
  </code></pre>
  
  <p>I want to import <strong>library</strong>/libdir into <strong>main</strong>/dir1 so that it looks like this:</p>
  
  <pre><code>/
      dir1
          some-file
      dir2
  </code></pre>
  
  <p>Using git-subtree, I can specify to import into <em>dir1</em> with the <code>--prefix</code> argument, but can I also specify to only take the contents of a specific directory in the subtree?</p>
  
  <p>The reason for using git-subtree is that I can later synchronize the two repositories.</p>