Stop new merge request page tabs updating the URL

The tabs on the merge_requests#new where changing the URL to URLs meant
for merge_requests#show. The URL don't actually exist on the new page,
so the simplest solution is to not update the URL for this page.

Closes #30419

app/views/projects/merge_requests/_new_submit.html.haml

@@ -53,5 +53,6 @@
 
 :javascript
   var merge_request = new MergeRequest({
-    action: "#{(@show_changes_tab ? 'new/diffs' : 'new')}"
+    action: "#{(@show_changes_tab ? 'new/diffs' : 'new')}",
+    setUrl: false,
   });

changelogs/unreleased/mr-new-page-changing-url.yml

+---
+title: Fixed tabs on new merge request page causing incorrect URLs
+merge_request:
+author: