Display base diff for merged merge requests

It should stay so until merge ref diff is persisted in the database

app/models/merge_request.rb

@@ -972,7 +972,7 @@ class MergeRequest < ApplicationRecord
   # rubocop: enable CodeReuse/ServiceClass
 
   def diffable_merge_ref?
-    merge_ref_head.present? && (Feature.enabled?(:display_merge_conflicts_in_diff, project) || can_be_merged?)
+    open? && merge_ref_head.present? && (Feature.enabled?(:display_merge_conflicts_in_diff, project) || can_be_merged?)
   end
 
   # Returns boolean indicating the merge_status should be rechecked in order to

spec/models/merge_request_spec.rb

@@ -4275,6 +4275,14 @@ RSpec.describe MergeRequest, factory_default: :keep do
           expect(subject.diffable_merge_ref?).to eq(true)
         end
 
+        context 'merge request is merged' do
+          subject { build_stubbed(:merge_request, :merged, project: project) }
+
+          it 'returns false' do
+            expect(subject.diffable_merge_ref?).to eq(false)
+          end
+        end
+
         context 'merge request cannot be merged' do
           before do
             subject.mark_as_unchecked!