Fix new branch button spec

1. We can create the note directly on the issue, rather than attaching
   it after creation.
2. We need to update the MergeRequestClosesIssues relation to ensure
   that the issue know that it's closed by the MR.
3. We should also check that the unavailable button is displayed - not
   just that the available button is displayed.

spec/features/issues/new_branch_button_spec.rb

@@ -18,8 +18,8 @@ feature 'Start new branch from an issue', feature: true do
     end
 
     context "when there is a referenced merge request" do
-      let(:note) do
-        create(:note, :on_issue, :system, project: project,
+      let!(:note) do
+        create(:note, :on_issue, :system, project: project, noteable: issue,
                                           note: "Mentioned in !#{referenced_mr.iid}")
       end
       let(:referenced_mr) do
@@ -28,12 +28,13 @@ feature 'Start new branch from an issue', feature: true do
       end
 
       before do
-        issue.notes << note
+        referenced_mr.cache_merge_request_closes_issues!(user)
 
         visit namespace_project_issue_path(project.namespace, project, issue)
       end
 
       it "hides the new branch button", js: true do
+        expect(page).to have_css('#new-branch .unavailable')
         expect(page).not_to have_css('#new-branch .available')
         expect(page).to have_content /1 Related Merge Request/
       end