Add feature-flag support

Returns error in MergeToRefService when
merge_to_tmp_merge_ref_path ff is disabled.

app/services/merge_requests/merge_to_ref_service.rb

@@ -34,7 +34,9 @@ module MergeRequests
 
     def error_check!
       error =
-        if !merge_method_supported?
+        if Feature.disabled?(:merge_to_tmp_merge_ref_path, project)
+          'Feature is not enabled'
+        elsif !merge_method_supported?
           "#{project.human_merge_method} to #{target_ref} is currently not supported."
         elsif !hooks_validation_pass?(merge_request)
           hooks_validation_error(merge_request)

spec/services/merge_requests/merge_to_ref_service_spec.rb

@@ -74,6 +74,15 @@ describe MergeRequests::MergeToRefService do
       process_merge_to_ref
     end
 
+    it 'returns error when feature is disabled' do
+      stub_feature_flags(merge_to_tmp_merge_ref_path: false)
+
+      result = service.execute(merge_request)
+
+      expect(result[:status]).to eq(:error)
+      expect(result[:message]).to eq('Feature is not enabled')
+    end
+
     it 'returns an error when the failing to process the merge' do
       allow(project.repository).to receive(:merge_to_ref).and_return(nil)