transaction_id merge tag

// add a {transaction_id} merge tag
add_filter( 'gform_replace_merge_tags', 'replace_transaction_id', 10, 7 );
function replace_transaction_id( $text, $form, $entry, $url_encode, $esc_html, $nl2br, $format ) {
    GFCommon::log_debug( "gform_replace_merge_tags(): \$entry => " . print_r( $entry, true ) );
    $text = str_replace( '{transaction_id}', $entry['transaction_id'], $text );
    return $text;
}