upload-pack: Implement no-done capability

upload-pack: Implement no-done capability

If the client requests both multi_ack_detailed and no-done then
upload-pack is free to immediately send a PACK following its first
'ACK %s ready' message.  The upload-pack response actually winds
up being:

  ACK %s common
  ... (maybe more) ...
  ACK %s ready
  NAK
  ACK %s
  PACK.... the pack stream ....

For smart HTTP connections this saves one HTTP RPC, reducing
the overall latency for a trivial fetch.  For git:// and ssh://
a no-done option slightly reduces latency by removing one
server->client->server round-trip at the end of the common
ancestor negotiation.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>