pkt-line: Make packet_read_line easier to debug

When there is an error parsing the 4 byte length component we now
display it as part of the die message, this may hint as to what
data was misunderstood by the application.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pkt-line.c

@@ -129,7 +129,7 @@ int packet_read_line(int fd, char *buffer, unsigned size)
 	safe_read(fd, linelen, 4);
 	len = packet_length(linelen);
 	if (len < 0)
-		die("protocol error: bad line length character");
+		die("protocol error: bad line length character: %.4s", linelen);
 	if (!len)
 		return 0;
 	len -= 4;