CodingGuidelines: update for C99
Since
7bc341e21b5 (git-compat-util: add a test balloon for C99
support, 2021-12-01) we've had a hard dependency on C99, but the prose
in CodingGuidelines was written under the assumption that we were
using C89 with a few C99 features.
As the updated prose notes we'd still like to hold off on novel C99
features, but let's make it clear that we target that C version, and
then enumerate new C99 features that are safe to use.
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>