'See object_index.
:param sha: A *binary* SHA string. (20 characters long)_'

def _object_index(self, sha):

    assert (len(sha) == 20)
    idx = ord(sha[:1])
    if (idx == 0):
        start = 0
    else:
        start = self._fan_out_table[(idx - 1)]
    end = self._fan_out_table[idx]
    i = bisect_find_sha(start, end, sha, self._unpack_name)
    if (i is None):
        raise KeyError(sha)
    return self._unpack_offset(i)
