Include HMR refresh hash in `"use cache"` cache keys (#75474)
Today, when editing server components that use `"use cache"` functions,
only updates to the uncached parts of the component will be reflected in
the rendered result after the HMR refresh was applied. Any cached
functions/components will show the same content as before the edit.
This is problematic when a function with a `"use cache"` directive is
edited. In this case, the edits won't be applied, and stale content is
shown in the browser, until the dev server is restarted.
With this PR, we are now including an HMR refresh hash in the cache key
for all `"use cache"` functions. This ensures that those functions are
revalidated when a server component is edited, which avoids showing
stale content.
However, while the `"use cache"` functions are revalidated during the
HMR refresh, we will still restore any fetches inside of `"use cache"`
from the [server components HMR
cache](https://nextjs.org/docs/app/api-reference/config/next-config-js/serverComponentsHmrCache)
(unless their input changed because of the edit).
For Webpack, a real content hash is used, which means that if an edit is
reverted to a prior edit, the previously cached data will be retrieved
from the cache. For Turbopack, the HMR hash is currently just an
incrementing counter, which may be improved in a follow-up PR.
closes NAR-65
