We now advertise the side-band-64k capability inside of ReceivePack,
allowing hooks to echo status messages down the side band channel
instead of over the optional stderr stream.

This change permits hooks running inside of an http:// based push
invocation to still message the end-user with more detailed errors
than the small per-command string in the status report.

Change-Id: I64f251ef2d13ab3fd0e1a319a4683725455e5244
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>