Fix rendering pipelines for a new merge request

What does this MR do?

This MR fixes a bug with invalid pipelines API call, which leads to raising an exception.

Does this MR meet the acceptance criteria?

• Changelog entry added
• Added tests for this feature/bug

What are the relevant issue numbers?

See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8844#note_22930365

spec/features/merge_requests/create_new_mr_spec.rb

@@ -84,4 +84,24 @@
     expect(page).not_to have_selector('#error_explanation')
     expect(page).not_to have_content('The form contains the following error')
   end
+
+  context 'when a new merge request has a pipeline' do
+    let!(:pipeline) do
+      create(:ci_pipeline, sha: project.commit('fix').id,
+                           ref: 'fix',
+                           project: project)
+    end
+
+    it 'shows pipelines for a new merge request' do
+      visit new_namespace_project_merge_request_path(
+        project.namespace, project,
+        merge_request: { target_branch: 'master', source_branch: 'fix' })
+
+      page.within('.merge-request') do
+        click_link 'Pipelines'
+
+        expect(page).to have_content "##{pipeline.id}"
+      end
+    end
+  end
 end