Improve tests for merge request diff model

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/models/merge_request_diff.rb

@@ -169,7 +169,7 @@ class MergeRequestDiff < ActiveRecord::Base
 
   def compare_with(sha, straight = true)
     # When compare merge request versions we want diff A..B instead of A...B
-    # so we handle cases when user squash and rebase commits in one of versions.
+    # so we handle cases when user does squash and rebase of the commits between versions.
     # For this reason we set straight to true by default.
     CompareService.new.execute(project, head_commit_sha, project, sha, straight)
   end

spec/models/merge_request_diff_spec.rb

@@ -74,6 +74,7 @@ describe MergeRequestDiff, models: true do
         end
       end
     end
+  end
 
   describe '#commits_sha' do
     shared_examples 'returning all commits SHA' do
@@ -96,5 +97,14 @@ describe MergeRequestDiff, models: true do
       it_behaves_like 'returning all commits SHA'
     end
   end
+
+  describe '#compare_with' do
+    subject { create(:merge_request).merge_request_diff }
+
+    it 'delegates compare to the service' do
+      expect(CompareService).to receive(:new).and_call_original
+
+      subject.compare_with('ae73cb07c9eeaf35924a10f713b364d32b2dd34f')
+    end
   end
 end