Show un-highlighted diffs when blobs are the same

For some old merge requests, we don't have enough information to figure out the
old blob and the new blob for the file. This means that we can't highlight the
diff correctly, but we can still display it without highlighting.

changelogs/unreleased/fix-old-mr-diffs.yml

+---
+title: Show un-highlighted text diffs when we do not have references to the correct
+  blobs
+merge_request:
+author:
+type: fixed

lib/gitlab/diff/file.rb

@@ -186,7 +186,10 @@ module Gitlab
       end
       def content_changed?
-        old_blob && new_blob && old_blob.id != new_blob.id
+        return blobs_changed? if diff_refs
+        return false if new_file? || deleted_file? || renamed_file?
+
+        text? && diff_lines.any?
       end
       def different_type?
@@ -225,6 +228,10 @@ module Gitlab
       private
+      def blobs_changed?
+        old_blob && new_blob && old_blob.id != new_blob.id
+      end
+
       def simple_viewer_class
         return DiffViewer::NotDiffable unless diffable?

spec/lib/gitlab/diff/file_spec.rb

@@ -15,6 +15,17 @@ describe Gitlab::Diff::File do
     it { expect(diff_lines.first).to be_kind_of(Gitlab::Diff::Line) }
   end
+  describe '#highlighted_diff_lines' do
+    it 'highlights the diff and memoises the result' do
+      expect(Gitlab::Diff::Highlight).to receive(:new)
+                                           .with(diff_file, repository: project.repository)
+                                           .once
+                                           .and_call_original
+
+      diff_file.highlighted_diff_lines
+    end
+  end
+
   describe '#mode_changed?' do
     it { expect(diff_file.mode_changed?).to be_falsey }
   end
@@ -122,8 +133,20 @@ describe Gitlab::Diff::File do
         let(:commit) { project.commit('2f63565e7aac07bcdadb654e253078b727143ec4') }
         let(:diff_file) { commit.diffs.diff_file_with_new_path('files/images/6049019_460s.jpg') }
-        it 'returns true' do
-          expect(diff_file.content_changed?).to be_truthy
+        context 'when the blobs are different' do
+          it 'returns true' do
+            expect(diff_file.content_changed?).to be_truthy
+          end
+        end
+
+        context 'when there are no diff refs' do
+          before do
+            allow(diff_file).to receive(:diff_refs).and_return(nil)
+          end
+
+          it 'returns false' do
+            expect(diff_file.content_changed?).to be_falsey
+          end
         end
       end
@@ -131,8 +154,20 @@ describe Gitlab::Diff::File do
         let(:commit) { project.commit('570e7b2abdd848b95f2f578043fc23bd6f6fd24d') }
         let(:diff_file) { commit.diffs.diff_file_with_new_path('files/ruby/popen.rb') }
-        it 'returns true' do
-          expect(diff_file.content_changed?).to be_truthy
+        context 'when the blobs are different' do
+          it 'returns true' do
+            expect(diff_file.content_changed?).to be_truthy
+          end
+        end
+
+        context 'when there are no diff refs' do
+          before do
+            allow(diff_file).to receive(:diff_refs).and_return(nil)
+          end
+
+          it 'returns true' do
+            expect(diff_file.content_changed?).to be_truthy
+          end
         end
       end
     end
@@ -278,6 +313,7 @@ describe Gitlab::Diff::File do
         allow(diff_file).to receive(:deleted_file?).and_return(false)
         allow(diff_file).to receive(:renamed_file?).and_return(false)
         allow(diff_file).to receive(:mode_changed?).and_return(false)
+        allow(diff_file).to receive(:raw_text?).and_return(false)
       end
       it 'returns a No Preview viewer' do