pkt-line: simplify starts_with checks in packet tracing

We carefully check that our pkt buffer has enough characters
before seeing if it starts with "PACK". The intent is to
avoid reading random memory if we get a short buffer like
"PAC".

However, we know that the traced packets are always
NUL-terminated. They come from one of these sources:

  1. A string literal.

  2. `format_packet`, which uses a strbuf.

  3. `packet_read`, which defensively NUL-terminates what we
     read.

We can therefore drop the length checks, as we know we will
hit the trailing NUL if we have a short input.

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

pkt-line.c

@@ -24,8 +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 ((len >= 4 && starts_with(buf, "PACK")) ||
-	    (len >= 5 && starts_with(buf+1, "PACK"))) {
+	if (starts_with(buf, "PACK") || starts_with(buf + 1, "PACK")) {
 		strbuf_addstr(&out, "PACK ...");
 		trace_disable(&trace_packet);
 	}