Update findHeadInCache to return entire Cache Node

For PPR, we will have two versions of the head: a prefetched version,
and a full dynamic version. In preparation for this, I've updated
findHeadInCache to return the entire Cache Node instead of just the
`head` field, so in the future we can render either `head`
or `prefetchHead`.

Author: acdlite

packages/next/src/client/components/app-router.tsx

@@ -542,9 +542,10 @@ function Router({
   const { cache, tree, nextUrl, focusAndScrollRef } =
     useUnwrapState(reducerState)
 
-  const head = useMemo(() => {
+  const headCacheNode = useMemo(() => {
     return findHeadInCache(cache, tree[1])
   }, [cache, tree])
+  const head = headCacheNode !== null ? headCacheNode.head : null
 
   let content = (
     <RedirectBoundary>

packages/next/src/client/components/router-reducer/reducers/find-head-in-cache.test.tsx

@@ -94,7 +94,7 @@ describe('findHeadInCache', () => {
 
     const result = findHeadInCache(cache, routerTree[1])
 
-    expect(result).toMatchObject(
+    expect(result?.head).toMatchObject(
       <>
         <title>About page!</title>
       </>

packages/next/src/client/components/router-reducer/reducers/find-head-in-cache.ts

@@ -5,10 +5,11 @@ import { createRouterCacheKey } from '../create-router-cache-key'
 export function findHeadInCache(
   cache: CacheNode,
   parallelRoutes: FlightRouterState[1]
-): React.ReactNode {
+): CacheNode | null {
   const isLastItem = Object.keys(parallelRoutes).length === 0
   if (isLastItem) {
-    return cache.head
+    // Returns the entire Cache Node of the segment whose head we will render.
+    return cache
   }
   for (const key in parallelRoutes) {
     const [segment, childParallelRoutes] = parallelRoutes[key]
@@ -30,5 +31,5 @@ export function findHeadInCache(
     }
   }
 
-  return undefined
+  return null
 }