sshpeer: also use doublepipe for client to server communication
This will allow even more real time output when the server issue output in the
middle a stream push.
# The 9diff extdiff command generates diff(1) compatible output
# suitable for use with the plumber(4).
[extensions]
extdiff =
[extdiff]
9diff = /bin/mercurial/9diff -cm $parent $child $root