Fix MR changes tab size count

This was wrong when there were over 100 files in the diff, because we
did not use the same diff options as subclasses of
`Gitlab::Diff::FileCollection::Base` when getting the raw diffs. (The
reason we don't use those classes directly is because they may perform
highlighting, which isn't needed for just counting the diffs.)

app/models/merge_request.rb

@@ -203,7 +203,11 @@ class MergeRequest < ActiveRecord::Base
   end
 
   def diff_size
-    opts = diff_options || {}
+    # The `#diffs` method ends up at an instance of a class inheriting from
+    # `Gitlab::Diff::FileCollection::Base`, so use those options as defaults
+    # here too, to get the same diff size without performing highlighting.
+    #
+    opts = Gitlab::Diff::FileCollection::Base.default_options.merge(diff_options || {})
 
     raw_diffs(opts).size
   end

changelogs/unreleased/fix-mr-size-with-over-100-files.yml

+---
+title: Fix MR changes tab size count when there are over 100 files in the diff
+merge_request:
+author:

spec/models/merge_request_spec.rb

@@ -209,6 +209,50 @@ describe MergeRequest, models: true do
     end
   end
 
+  describe '#diff_size' do
+    let(:merge_request) do
+      build(:merge_request, source_branch: 'expand-collapse-files', target_branch: 'master')
+    end
+
+    context 'when there are MR diffs' do
+      before do
+        merge_request.save
+      end
+
+      it 'returns the correct count' do
+        expect(merge_request.diff_size).to eq(105)
+      end
+
+      it 'does not perform highlighting' do
+        expect(Gitlab::Diff::Highlight).not_to receive(:new)
+
+        merge_request.diff_size
+      end
+    end
+
+    context 'when there are no MR diffs' do
+      before do
+        merge_request.compare = CompareService.new(
+          merge_request.source_project,
+          merge_request.source_branch
+        ).execute(
+          merge_request.target_project,
+          merge_request.target_branch
+        )
+      end
+
+      it 'returns the correct count' do
+        expect(merge_request.diff_size).to eq(105)
+      end
+
+      it 'does not perform highlighting' do
+        expect(Gitlab::Diff::Highlight).not_to receive(:new)
+
+        merge_request.diff_size
+      end
+    end
+  end
+
   describe "#related_notes" do
     let!(:merge_request) { create(:merge_request) }