CodingGuidelines: spell out how we use grep in our scripts

CodingGuidelines: spell out how we use grep in our scripts

Our scripts try to stick to fairly limited subset of POSIX BRE for
portability.  It is unclear from manual page from GNU grep which is GNU
extension and which is portable, so let's spell it out to help new people
to keep their contributions from hurting porters.

Signed-off-by: Junio C Hamano <gitster@pobox.com>