title: python-git checkout file in repo
body: <p>I'm trying to use python-git to update a single file (checkout) in the repository from remote. 
  When doing this with command line one would probably use <code>git checkout &lt;file&gt;</code> but I haven't found a proper way of doing this in python-git? 
  So far I was only able to do a pull on the entire repo, what I want to limit this to one file only</p>
  
  <pre><code>from git import Repo
  
  repo = Repo('./')
  origin = repo.remote()
  ret = origin.pull()
  </code></pre>