'Get a single (git) commit.
:param str sha: (required), sha of the commit
:returns: :class:`Commit <github3.git.Commit>` if successful,
otherwise None'

def git_commit(self, sha):

    json = {}
    if sha:
        url = self._build_url(u'git', u'commits', sha, base_url=self._api)
        json = self._json(self._get(url), 200)
    return self._instance_or_null(Commit, json)
