Tries to pull changes from external location.

def pull(self, url):
        """
        Tries to pull changes from external location.
        """
        url = self._get_url(url)
        cmd = ['pull']
        cmd.append("--ff-only")
        cmd.append(url)
        cmd = ' '.join(cmd)
        # If error occurs run_git_command raises RepositoryError already
        self.run_git_command(cmd)