receive-pack: Send hook output over side band #2

receive-pack: Send hook output over side band #2

If the client requests to enable side-band-64k capability we can
safely send any hook stdout or stderr data down side band #2,
so the client can present it to the user.

If side-band-64k isn't enabled, hooks continue to inherit stderr
from the parent receive-pack process.

When the side band channel is being used the push client will wind up
prefixing all server messages with "remote: ", just like fetch does,
so our test vector has to be updated with the new expected output.

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