CodingGuidelines: mention "static" and "extern"

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>