Fixed source projects not filtering

Filtering source projects in the merge request creation form
would show a flash error because it is trying to filter remotely
to an undefined URL.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/52728

app/assets/javascripts/compare_autocomplete.js

@@ -40,7 +40,7 @@ export default function initCompareAutocomplete(limitTo = null, clickHandler = (
       },
       selectable: true,
       filterable: true,
-      filterRemote: true,
+      filterRemote: !!$dropdown.data('refsUrl'),
       fieldName: $dropdown.data('fieldName'),
       filterInput: 'input[type="search"]',
       renderRow: function(ref) {

changelogs/unreleased/mr-creation-source-project-filtering.yml

+---
+title: Fixed source project not filtering in merge request creation compare form
+merge_request:
+author:
+type: fixed

spec/features/merge_request/user_creates_mr_spec.rb

@@ -28,4 +28,29 @@ describe 'Merge request > User creates MR' do
       it_behaves_like 'a creatable merge request'
     end
   end
+
+  context 'source project', :js do
+    let(:user)        { create(:user) }
+    let(:target_project) { create(:project, :public, :repository) }
+    let(:source_project) { target_project }
+
+    before do
+      source_project.add_maintainer(user)
+
+      sign_in(user)
+      visit project_new_merge_request_path(
+        target_project,
+        merge_request: {
+          source_project_id: source_project.id,
+          target_project_id: target_project.id
+        })
+    end
+
+    it 'filters source project' do
+      find('.js-source-project').click
+      find('.dropdown-source-project input').set('source')
+
+      expect(find('.dropdown-source-project .dropdown-content')).not_to have_content(source_project.name)
+    end
+  end
 end