upload_pack(
git_dir,body) ->response
Answer an upload-pack POST body with an upload-pack response
Stateless-RPC negotiation: rounds without done get a bare NAK (all
haves are ignored); once the client sends done, a full pack of
everything reachable from the wants follows, side-band framed when the
client requested it. Errors become ERR pkts / band-3 messages.