git-compat-util.h: implement a different ARRAY_SIZE macro for for safely deriving the size of array

To get number of elements in an array git use the ARRAY_SIZE macro
defined as:

       #define ARRAY_SIZE(x) (sizeof(x)/sizeof((x)[0]))

The problem with it is a possibility of mistakenly passing to it a
pointer instead an array. The ARRAY_SIZE macro as conventionally
defined does not provide good type-safety and the open-coded
approach is more fragile, more verbose and provides no improvement in
type-safety.

Use instead a different but compatible ARRAY_SIZE() macro,
which will also break compile if you try to
use it on a pointer. This implemention revert to the original code
if the compiler doesn't know the typeof and __builtin_types_compatible_p
GCC extensions.

This can ensure our code is robust to changes, without
needing a gratuitous macro or constant. A similar
ARRAY_SIZE implementation also exists in the linux kernel.

Credits to Rusty Russell and his ccan library.

Signed-off-by: Elia Pinto <gitter.spiros@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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