Fix the revalidation condition of the first page

[shuding]

This PR fixes a useSWRInfinite bug introduced in #799 by me (thanks @pacocoursey for reporting). To reproduce, just simply use refreshInterval with useSWRInfinite, and the first page will never be revalidated:

Reproduction: https://codesandbox.io/s/swr-infinite-forked-mi7rf?file=/src/App.js

Postmortem and Fix

The cause of the bug is this LOC:

(typeof force === 'undefined' && i === 0 && originalData) ||

...to determine if we should fetch this page or not, which can break down into:

• typeof force === 'undefined: It's not a force revalidation, like mutate().
• i === 0: It's the first page, we don't want to revalidate every page upon focus.
• originalData: It's not the first render. This is the mistake.

What we want to achieve for useSWRInfinite is this experience: If we have the cache and the screen is empty (first render), we display the cache ASAP. So we added the third condition above. But we should check dataRef instead of originalData (originalData is specifically for mutations).

Also added another test.

[codesandbox-ci]

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Latest deployment of this branch, based on commit ed9e2d1:

Sandbox	Source
SWR-Basic	Configuration
SWR-States	Configuration
SWR-Infinite	Configuration
SWR-Infinite (forked)	PR

[huozhi]

lgtm from test case perspective