Forward the renderer's already-pinned {mergeBase, headOid} to the SSH relay so a single-file branch diff reads the two blobs directly instead of rediscovering live HEAD. Six sequential git processes become two concurrent reads, and a branch move mid-review no longer changes which revision is displayed.
Equivalence with the legacy route is proven against real Git across 14 change types; wire compatibility is proven over a real SSH socket against relay bundles built from main and from the pre-merge-base.
