Implemented fragment matcher for Issuable

app/assets/javascripts/sidebar/fragmentTypes.json

+{"__schema":{"types":[{"kind":"UNION","name":"Issuable","possibleTypes":[{"name":"Issue"},{"name":"MergeRequest"}]}, {"kind":"INTERFACE","name":"User","possibleTypes":[{"name":"UserCore"}]}]}}

app/assets/javascripts/sidebar/graphql.js

+import { IntrospectionFragmentMatcher } from 'apollo-cache-inmemory';
 import VueApollo from 'vue-apollo';
 import createDefaultClient from '~/lib/graphql';
+import introspectionQueryResultData from './fragmentTypes.json';
 
-export const defaultClient = createDefaultClient();
+const fragmentMatcher = new IntrospectionFragmentMatcher({
+  introspectionQueryResultData,
+});
+
+export const defaultClient = createDefaultClient(
+  {},
+  {
+    cacheConfig: {
+      fragmentMatcher,
+    },
+    assumeImmutableResults: true,
+  },
+);
 
 export const apolloProvider = new VueApollo({
   defaultClient,