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>