title: pygit2 clone_repository authentication required but no callback set
body: <p>I'm trying to clone a repository from stash using an ssh link.  I get an error saying authentication is required, This should not require a username and password.  How do I fix this error?</p>
  
  <pre><code>from pygit2 import *
  repo_name = "ssh://git@stash:TheProject"
  clone_repository(self.repo_name, self.repo_dir, credentials=cred)
  
  Traceback (most recent call last):
  File "/usr/lib/python2.7/lib-tk/Tkinter.py", line 1413, in __call__ return self.func(*args)
  File "rio_telem_tool.py", line 143, in build clone_repository(self.repo_name, self.repo_dir)
  File "/usr/local/lib/python2.7/dist-packages/pygit2/__init__.py", line 265, in clone_repository check_error(err)
  File "/usr/local/lib/python2.7/dist-packages/pygit2/errors.py", line 56, in check_error raise GitError(message)
  GitError: authentication required but no callback set
  </code></pre>