CodingGuidelines: clarify C #include rules

Even though "advice.h" includes "git-compat-util.h", it is not sensible to have it as the first #include and indirectly satisify the "You must give git-compat-util.h a clean environment to set up feature test macros before including any of the system headers are included", which is the real requirement. Because: - A command that interacts with the object store, config subsystem, the index, or the working tree cannot do anything without using what is declared in "cache.h"; - A built-in command must be declared in "builtin.h", so anything in builtin/*.c must include it; - These two headers both include "git-compat-util.h" as the first thing; and - Almost all our *.c files (outside compat/ and borrowed files in xdiff/) need some Git-ness from "cache.h" to do something Git-ish. let's explicitly specify that one of these three header files must be the first thing that is included. Any of our *.c file should include the header file that directly declares what it uses, instead of relying on the fact that some *.h file it includes happens to include another *.h file that declares the necessary function or type. Spell it out as another guideline item. Helped-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>