test-lib-functions: make packetize() more efficient

test-lib-functions: make packetize() more efficient

The packetize() function takes its input on stdin, and requires 4
separate sub-processes to format a simple string. We can do much better
by getting the length via the shell's "${#packet}" construct. The one
caveat is that the shell can't put a NUL into a variable, so we'll have
to continue to provide the stdin form for a few calls.

There are a few other cleanups here in the touched code:

 - the stdin form of packetize() had an extra stray "%s" when printing
   the packet

 - the converted calls in t5562 can be made simpler by redirecting
   output as a block, rather than repeated appending

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