title: git-subtree: Push changes from an cloned repo
body: <p>I'm using git-subtree(from Avery Pennarun). In my current git repo I have of course
  all my project files/folders and a subtree called "lib".
  If I now clone this git repo using <code>git clone</code> I get all of the project files and
  the subtree "lib" (everything as it should be).
  What I tried now: I changed something within the subtree "lib" in the cloned
  repo and tried to push the changes back to the remote repo of the subtree "lib" using
  <code>git subtree push</code>, but it didn't work. What is the problem? Do I have to add it
  as subtree first with git subtree add?</p>
  
  <p>Thx in advance</p>