private function resolvePull($user, $repo, $reference)
    {
        if (!$pull = $this->parseRefAsPull($reference)) {
            return null;
        }

        $api = $this->client->api('pull_request');
        $params = [$user, $repo, $pull];

        $data = $this->callGitHub([$api, 'show'], $params);
        return $data['head']['sha'] ?? null;
    }