Always return a simple diff viewer

We didn't have a fallback case before, because we believed the conditions were
exhaustive. They weren't, so we can always fallback to not previewing.

lib/gitlab/diff/file.rb

@@ -250,6 +250,8 @@ module Gitlab
           DiffViewer::Renamed
         elsif mode_changed?
           DiffViewer::ModeChanged
+        else
+          DiffViewer::NoPreview
         end
       end

spec/lib/gitlab/diff/file_spec.rb

@@ -270,6 +270,20 @@ describe Gitlab::Diff::File do
         expect(diff_file.simple_viewer).to be_a(DiffViewer::ModeChanged)
       end
     end
+
+    context 'when no other conditions apply' do
+      before do
+        allow(diff_file).to receive(:content_changed?).and_return(false)
+        allow(diff_file).to receive(:new_file?).and_return(false)
+        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)
+      end
+
+      it 'returns a No Preview viewer' do
+        expect(diff_file.simple_viewer).to be_a(DiffViewer::NoPreview)
+      end
+    end
   end
   describe '#rich_viewer' do