'Check if SHA-1 is the same as remote branch
See: https://git.io/vaqIw
:param str differs_from: (optional), sha to compare against
:returns: string of the SHA or None'

def latest_sha(self, differs_from=u''):

    headers = {u'Accept': u'application/vnd.github.chitauri-preview+sha', u'If-None-Match': u'"{0}"'.format(differs_from)}
    base = self._api.split(u'/branches', 1)[0]
    url = self._build_url(u'commits', self.name, base_url=base)
    resp = self._get(url, headers=headers)
    if self._boolean(resp, 200, 304):
        return resp.content
    return None
