CodingGuidelines: mention "static" and "extern"

It perhaps goes without saying that file-local stuff should be marked static, but it does not hurt to remind people. Less obvious is that we are settling on "do not include extern in function declarations". It is already the default unless the function was previously declared static (but if you are following a static declaration with an unmarked one, you should think about why you are declaring the thing twice). And so it just becomes an extra noise-word in our header files. We used to give the opposite advice, so there are quite a few "extern" markers in early Git code. But this at least makes a concrete suggestion that we can follow going forward. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>