git replace substitutes one object for another via refs/replace/<orig-sha>. Niche feature — used for history grafts and commit rewriting without invalidating SHAs downstream.

Scope

• ObjectResolver.read honors replace refs transparently when replace_refs: true (default like git).
• Ref API supports creating replace refs.
• Capability advertisement so clients know we support it.

Acceptance

• A replace ref mapping commit A → commit B causes git log on our server to show B’s content when walking A.
• Works end-to-end with git replace client-side.

(Low priority; filing so we don’t forget it exists.)