test-lib-functions: simplify packetize() stdin code

The code path in packetize() for reading stdin needs to handle NUL
bytes, so we can't rely on shell variables. However, the current code
takes a whopping 4 processes and uses a temporary file. We can do this
much more simply and efficiently by using a single perl invocation (and
we already rely on perl in the matching depacketize() function).

We'll keep the non-stdin code path as it is, since that uses zero extra
processes.

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

t/test-lib-functions.sh

@@ -1373,11 +1373,10 @@ packetize() {
 		packet="$*"
 		printf '%04x%s' "$((4 + ${#packet}))" "$packet"
 	else
-		cat >packetize.tmp &&
-		len=$(wc -c <packetize.tmp) &&
-		printf '%04x' "$(($len + 4))" &&
-		cat packetize.tmp &&
-		rm -f packetize.tmp
+		perl -e '
+			my $packet = do { local $/; <STDIN> };
+			printf "%04x%s", 4 + length($packet), $packet;
+		'
 	fi
 }