CodingGuidelines: allow ${#posix} == strlen($posix)

The construct has been in POSIX for the past 10+ years, and we have used in t9xxx (subversion) series of the tests, so we know it is at portable across systems that people have run those tests, which is almost everything we'd care about. Let's loosen the rule; luckily, the check-non-portable-shell script does not have any rule to find its use, so the only change needed is a removal of one paragraph from the documentation. Helped-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>