title: GitPython error when pulling from remote repository
body: <p>I'm trying to pull from a remote repo in a local one with this code:</p>
  
  <pre><code>repo = git.Repo('/home/user/repo/')
  o = repo.remotes.origin
  try:
      o.pull()
  except:
      logging.exception("oops:")
  </code></pre>
  
  <p>This fails miserably with the following traceback:</p>
  
  <pre><code>Traceback (most recent call last):
    File "/home/user/my_site/app/app.py", line 58, in regen_logs
      o.pull()
    File "/home/user/my_site/venv/lib/python2.7/site-packages/git/remote.py", line 665, in pull
      proc = self.repo.git.pull(self, refspec, with_extended_output=True, as_process=True, v=True, **kwargs)
    File "/home/user/my_site/venv/lib/python2.7/site-packages/git/cmd.py", line 440, in &lt;lambda&gt;
      return lambda *args, **kwargs: self._call_process(name, *args, **kwargs)
    File "/home/user/my_site/venv/lib/python2.7/site-packages/git/cmd.py", line 834, in _call_process
      return self.execute(make_call(), **_kwargs)
    File "/home/user/my_site/venv/lib/python2.7/site-packages/git/cmd.py", line 576, in execute
      raise GitCommandNotFound(str(err))
  GitCommandNotFound: [Errno 2] No such file or directory
  </code></pre>
  
  <p>However, when doing the exact three commands (<code>Repo</code>, set origin, <code>pull()</code>) in an interactive session, it works just fine:</p>
  
  <pre><code>&gt;&gt;&gt; import git
  &gt;&gt;&gt; repo = git.Repo('/home/user/repo')
  &gt;&gt;&gt; o = repo.remotes.origin
  &gt;&gt;&gt; o.pull()
  [&lt;git.remote.FetchInfo object at 0x242d2b8&gt;]
  </code></pre>
  
  <p>I do have Git installed on the system:</p>
  
  <pre><code>$ rpm -q git
  git-1.8.3.1-6.el7.x86_64
  </code></pre>
  
  <p>Any clues what I'm doing wrong here?</p>