Tries to pull changes from external location.

def fetch(self, url):
        """
        Tries to pull changes from external location.
        """
        url = self._get_url(url)
        so, se = self.run_git_command('ls-remote -h %s' % url)
        refs = []
        for line in (x for x in so.splitlines()):
            sha, ref = line.split('\t')
            refs.append(ref)
        refs = ' '.join(('+%s:%s' % (r, r) for r in refs))
        cmd = '''fetch %s -- %s''' % (url, refs)
        self.run_git_command(cmd)