[code_review] Add generated synthetic examples from past bugs for RAG

We can generate synthetic comments (and synthetic CoT, #4585) from bug fixes: take a patch that fixes a bug, go back to the patch that introduced the bug, generate a synthetic review comment about the bug in the patch that introduced the bug (example at https://chatgpt.com/share/6709afeb-c86c-8002-a9cd-26a73321bb05).

This should help the LLM identify more potential bugs.