upload-pack/fetch-pack: support side-band communication

This implements a protocol extension between fetch-pack and
upload-pack to allow stderr stream from upload-pack (primarily
used for the progress bar display) to be passed back.

Signed-off-by: Junio C Hamano <junkio@cox.net>

pkt-line.h

@@ -8,5 +8,6 @@ void packet_flush(int fd);
 void packet_write(int fd, const char *fmt, ...) __attribute__((format (printf, 2, 3)));
 
 int packet_read_line(int fd, char *buffer, unsigned size);
+ssize_t safe_write(int, const void *, ssize_t);
 
 #endif