'Get a single (repo) commit.
See :func:`git_commit` for the Git Data Commit.
:param str sha: (required), sha of the commit
:returns: :class:`RepoCommit <github3.repos.commit.RepoCommit>` if
successful, otherwise None'

def commit(self, sha):

    url = self._build_url(u'commits', sha, base_url=self._api)
    json = self._json(self._get(url), 200)
    return self._instance_or_null(RepoCommit, json)
