receive-pack: Send internal errors over side-band #2

receive-pack: Send internal errors over side-band #2

If the client has requested side-band-64k capability, send any
of the internal error or warning messages in the muxed side-band
stream using the same band as our hook output, band #2.  By putting
everything in one stream we ensure all messages are processed by
the side-band demuxer, avoiding interleaving between our own stderr
and the side-band demuxer's stderr buffers.

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