t/lib-*.sh: drop executable bit

There's no need for shell libraries to have the executable bit. They're meant to be sourced, and running them stand-alone is pointless. Let's reduce any possible confusion by making it more clear they're not meant to be run this way. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>