Adds a few commit related assertions

spec/services/merge_requests/merge_service_spec.rb

@@ -48,12 +48,20 @@ describe MergeRequests::MergeService do
       end
 
       it 'is idempotent' do
+        repository = project.repository
+        commit_count = repository.commit_count
+        merge_commit = merge_request.merge_commit.id
+
         # a first invocation of execute is performed on the before block
         service.execute(merge_request)
 
         expect(merge_request.merge_error).to be_falsey
         expect(merge_request).to be_valid
         expect(merge_request).to be_merged
+
+        expect(repository.commits_by(oids: [merge_commit]).size).to eq(1)
+        expect(repository.commit_count).to eq(commit_count)
+        expect(merge_request.in_progress_merge_commit_sha).to be_nil
       end
 
       context 'when squashing' do