Make sure mutate invalidates concurrent requests

[shuding]

X-ref: nandorojo/swr-react-native#18

We need to make sure that mutate() invalidates concurrent requests and deduplicates them. It’ll be similar to this test case, but without a target value (only revalidates):

swr/test/use-swr-local-mutation.test.tsx

Lines 361 to 384 in d45b05d

	it('should ignore in flight requests when mutating', async () => {
	const key = createKey()
	// set it to 1
	act(() => {
	globalMutate(key, 1)
	})
	function Section() {
	const { data } = useSWR(key, () => createResponse(2, { delay: 150 }))
	return <div>{data}</div>
	}
	renderWithGlobalCache(<Section />)
	screen.getByText('1') // directly from cache
	await act(() => sleep(100)) // still suspending
	act(() => {
	globalMutate(key, 3)
	}) // set it to 3. this will drop the ongoing request
	await screen.findByText('3')
	await act(() => sleep(100))
	screen.getByText('3')
	})

[shuding]

It turns to be introduced in #1498 which I think should be the correct behavior, that mutate() will not be deduplicated by default because it was triggered explicitly by the user. So if calling two mutate()s in a row:

mutate()
mutate()

It's expected to have 2 revalidation requests started.

However requests fired with useSWR(key, fetcher) should be deduplicated, if we do mutate(). This is working as expected.

I think the solution here is to have a way to manually revalidate with deduplication enabled. We'll have to come up with a new option for that...

[pke]

@shuding But what if you have 2 components that run on a timer and use mutate to trigger a revalidate? Then we will have more than one request running, which does not make much sense. Even when the revalidate is user triggered, it should deduplicate. Otherwise this is highly confusing and not (!) documented.