pkt-line: tighten sideband PACK check when tracing

To find the start of the pack data, we accept the word PACK
at the beginning of any sideband channel, even though what
we really want is to find the pack data on channel 1. In
practice this doesn't matter, as sideband-2 messages tend to
start with "error:" or similar, but it is a good idea to be
explicit (especially as we add more code in this area, we
will rely on this assumption).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pkt-line.c

@@ -24,7 +24,7 @@ static void packet_trace(const char *buf, unsigned int len, int write)
 	strbuf_addf(&out, "packet: %12s%c ",
 		    packet_trace_prefix, write ? '>' : '<');
 
-	if (starts_with(buf, "PACK") || starts_with(buf + 1, "PACK")) {
+	if (starts_with(buf, "PACK") || starts_with(buf, "\1PACK")) {
 		strbuf_addstr(&out, "PACK ...");
 		trace_disable(&trace_packet);
 	}