send-pack: demultiplex a sideband stream with status data

send-pack: demultiplex a sideband stream with status data

If the server advertises side-band-64k capability, we request
it and pull the status report data out of side band #1, and let
side band #2 go to our stderr.  The latter channel be used by the
remote side to send our user messages.  This basically mirrors the
side-band-64k capability in upload-pack.

Servers may choose to use side band #2 to send error messages from
hook scripts that are meant for the push end user.

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