Enable `assumeImmutableResults` in project merge options

This commit solves https://gitlab.com/gitlab-org/gitlab/-/issues/338568
It enables `assumeImmutableResults` in the project merge options.

ee/app/assets/javascripts/pages/projects/edit/merge_options.js

@@ -46,7 +46,8 @@ const initMergeOptions = (mergePipelinesEnabled, mergeTrainsEnabled) => {
 const fetchMergeOptions = () => {
   const containerEl = document.querySelector('#project-merge-options');
   const { projectFullPath } = containerEl.dataset;
-  const defaultClient = createDefaultClient();
+
+  const defaultClient = createDefaultClient({}, { assumeImmutableResults: true });
 
   return defaultClient
     .query({