enable walking up route hierarchy to trigger onEnter

[nathanielks]

I discovered the onEnter route hook isn't triggered whenever navigating back up the route hierarchy. This enables triggering onEnter when navigating from a child branch to a parent branch.

For example, let's take the following routes:

• /users/:userId
• /users/:userId/assignments/:assignmentId

Navigating from the latter to the former would not trigger onEnter when entering the route. This PR alleviates that problem! It will trigger onEnter when you enter a parent route from a child route and will not re-trigger it if you click it again.

[taion]

That's intentional – you're not re-entering the parent route. This will be handled by onChange per #3108.

[nathanielks]

👌