fix: use live remote info for push up-to-date check; restore CHUNK_OBJECTS=400

When the remote repo is empty (or was wiped and recreated), the push command was falling back to a stale locally-cached tracking ref and incorrectly reporting "Everything up to date", sending nothing.

Fix: when GET /refs succeeds (remote_info is not None), use only the live branch_heads to determine the remote head. Only fall back to the cached tracking ref when the remote was unreachable.

Also restores CHUNK_OBJECTS from 100 back to 400 — the correct value for the production Nginx 300s timeout window.