Problem

Upload-pack uses NAK-only negotiation. The server always sends a full packfile even when the client already has most objects. The multi_ack_detailed capability enables incremental negotiation to minimize transfer size.

Impact

• Fetch operations transfer far more data than necessary
• Particularly wasteful for frequent small fetches (CI, polling)
• Standard git clients prefer multi_ack_detailed when available

Acceptance Criteria

• Upload-pack advertises multi_ack_detailed capability
• ACK/NAK/ACK common/ACK ready protocol implemented per git docs
• Packfile only contains objects the client doesn’t have
• Interop test: git fetch from a repo with shared history transfers minimal data
• Backward compatible: NAK-only still works for clients that don’t request it