src/vs/base/common/history.ts/HistoryNavigator2/has
class HistoryNavigator2:	has(t: T): boolean {
		let temp: HistoryNode<T> | undefined = this.head;
		while (temp) {
			if (temp.value === t) {
				return true;
			}
			temp = temp.next;
		}
		return false;
	}