Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/340387

**Problem**

HTML::Pipeline executes a chain of ReferenceFilter. Each filter with
reference cache tries to fetch parents for references and converts
Nokogiri structure into HTML document. This process consumes extra
memory and increases response time.

**Solution**

Use shared `result` variable to store rendered HTML. ReferenceCache
will use it when present or populate when missing.