func shouldVisit(rel string, mode Mode, updateRels map[string]bool) bool {
	if mode != UpdateDirsMode {
		return true
	}
	_, ok := updateRels[rel]
	return ok
}
func shouldVisit(rel string, mode Mode, updateRels map[string]bool) bool {
	if mode != UpdateDirsMode {
		return true
	}
	_, ok := updateRels[rel]
	return ok
}
// shouldVisit returns true if Walk should visit the subdirectory rel.