app-router: fix stale cache handling (#49160)

There was a bug reported in #42991 (comment) that would hang on load. 

After investigation, I noticed that it was because the cache node never applied the flight payload. It was because the requested flight payload was too deep into the tree whereas it should have been fetched from higher up in the tree because we specifically discarded that branch of the tree because it was stale.

packages/next/src/client/components/router-reducer/reducers/navigate-reducer.ts

@@ -288,7 +288,8 @@ export function navigateReducer(
           currentCache,
           flightSegmentPath,
           treePatch,
-          () => fetchServerResponse(url, newTree!, state.nextUrl)
+          // eslint-disable-next-line no-loop-func
+          () => fetchServerResponse(url, currentTree, state.nextUrl)
         )
       }