walk/walk.go
func shouldUpdate(rel string, mode Mode, updateParent bool, updateRels map[string]bool) bool {
	return mode == VisitAllUpdateSubdirsMode && updateParent || updateRels[rel]
}
func shouldUpdate(rel string, mode Mode, updateParent bool, updateRels map[string]bool) bool {
	return mode == VisitAllUpdateSubdirsMode && updateParent || updateRels[rel]
}
// shouldUpdate returns true if Walk should pass true to the callback's update
// parameter in the directory rel. This indicates the build file should be
// updated.