add optimistic UI

Author: rwieruch

src/App.js

@@ -122,6 +122,18 @@ const Watch = ({
         ? VIEWER_SUBSCRIPTIONS.UNSUBSCRIBED
         : VIEWER_SUBSCRIPTIONS.SUBSCRIBED,
     }}
+    optimisticResponse={{
+      updateSubscription: {
+        __typename: 'Mutation',
+        subscribable: {
+          __typename: 'Repository',
+          id,
+          viewerSubscription: isWatch(viewerSubscription)
+            ? VIEWER_SUBSCRIPTIONS.UNSUBSCRIBED
+            : VIEWER_SUBSCRIPTIONS.SUBSCRIBED,
+        },
+      },
+    }}
     update={updateWatch}
   >
     {(updateSubscription, { data, loading, error }) => (