Re: [PATCH] push: Use sideband channel for hook messages

"Shawn O. Pearce" [off-list ref] writes:

Rather than sending hook messages over stderr, and losing them
entirely on git:// and smart HTTP transports, receive-pack now
puts them onto a multiplexed sideband channel if the send-pack
client asks for the side-band-64k capablity.  This ensures that
hooks from the server can report their detailed error messages,
if any, no matter what git-aware transport is being used.

When the side band channel is being used the push client will wind up
prefixing all server messages with "remote: ", just like fetch does.

Signed-off-by: Shawn O. Pearce <redacted>

This feels to me a topic that has multiple unrelated changes mixed
together, making it unnecessarily confusing to review.

 - capabilities_to_send -> send_capabilities;

 - use of sideband in communication from send-pack to receive-pack;

 - adding communication in the opposite direction to async infrastructure;

 - use_sideband that is a boolean but is also used to hold
   LARGE_PACKET_MAX (on the receiving end).