title: Get HEAD revision number of SVN Repository with PYSVN
body: <p>I'm using pysvn to monitor the changes in a Subversion directory.
  This is how i get informations from revisions:</p>
  
  <pre><code>(...)
  svn_root = "http://svn/"
  client = pysvn.Client()
  from_revision = pysvn.Revision(pysvn.opt_revision_kind.number, 1500)
  to_revision = pysvn.Revision( pysvn.opt_revision_kind.head )
  
  revisions = client.log(svn_root, to_revision, to_revision, discover_changed_paths=True)
  </code></pre>
  
  <p>Now I want to get the changes not from a specific revision, like in my example, but the changes within the last 5 revisions (from <code>head - 5</code> to <code>head</code>). How can I accomplish that? How can i get the NUMBER of the head revision?</p>
  
  <p>I could do it by calling the Shell from Python. But I guess that there is a "Pythonic" way for that using pysvn.</p>