The sideband demultiplexor writes the data it receives on sideband
with xwrite().  We can lose data if the underlying write(2) results
in a short write.

If they are limited to unimportant bytes like eye-candy progress
meter, it may be OK to lose them, but lets be careful and ensure
that we use write_in_full() instead.  Note that the original does
not check for errors, and this rewrite does not check for one.  At
least not yet.

Signed-off-by: Junio C Hamano <gitster@pobox.com>