improve handling of sideband message display

improve handling of sideband message display

Currently the code looks for line break characters in order to prepend
"remote: " to every line received as many lines can be sent in a single
chunk.  However the opposite might happen too, i.e. a single message
line split amongst multiple chunks.  This patch adds support for the
later case to avoid displays like:

	remote: Compressing objeremote: cts: 100% (313/313), done.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>