Avoid using stale last.result in getCurrentResult.

This fixes #7978 in a better way than #8422, by actually checking
whether the `last.variables` used for `last.result` are still equal to
the current `options.variables`, before reusing `last.result` in
`ObservableQuery#getCurrentResult`.

src/core/ObservableQuery.ts

@@ -183,7 +183,8 @@ export class ObservableQuery<
   }
 
   public getCurrentResult(saveAsLastResult = true): ApolloQueryResult<TData> {
-    const lastResult = this.getLastResult();
+    // Use the last result as long as the variables match this.variables.
+    const lastResult = this.getLastResult(true);
 
     const networkStatus =
       this.queryInfo.networkStatus ||

src/core/__tests__/ObservableQuery.ts

@@ -798,7 +798,7 @@ describe('ObservableQuery', () => {
           expect(result.errors).toEqual([error]);
           expect(observable.getCurrentResult().errors).toEqual([error]);
           observable.setVariables(differentVariables);
-          expect(observable.getCurrentResult().errors).toEqual([error]);
+          expect(observable.getCurrentResult().errors).toBeUndefined();
         } else if (handleCount === 2) {
           expect(result.data).toEqual(dataTwo);
           expect(observable.getCurrentResult().data).toEqual(