title: Git subtree -- sharing subtrees with contributors
body: <p>In some of the repositories that I'm working with, I found git subtree does not behave as expected when sharing repos.</p>
  
  <p>Here's the scenario:</p>
  
  <ol>
  <li>I own main repo X and push it to origin.</li>
  <li>My colleague pulls repo X from origin, adds a subtree pointing to another repo Y, makes the commit, and pushes X back to origin.</li>
  <li>I pull X and see the changes my colleague has made. However, in my local repo, the folder containing subtree Y is just another file. I can't pull changes from repo Y even after adding repo Y as a remote. Git subtree always says that the folder already exists whenever I do a <code>git subtree add</code> and says that it can not find the commit when I do <code>git subtree merge</code>.</li>
  </ol>
  
  <p>Does anyone know the best way to deal with this situation? Ideally I would like to also be able to pull changes from repo Y even though I am not the user adding repo Y as a subtree of repo X. Is this possible? If not, what is the best way to share repositories having subtrees?</p>