add DUP_ARRAY

Add a macro for allocating and populating a shallow copy of an array.
It is intended to replace a sequence like this:

   ALLOC_ARRAY(dst, n);
   COPY_ARRAY(dst, src, n);

With the less repetitve:

   DUP_ARRAY(dst, src, n);

It checks whether the types of source and destination are compatible to
ensure the copy can be used safely.

An easier alternative would be to only consider the source and return
a void pointer, that could be used like this:

   dst = ARRAY_DUP(src, n);

That would be more versatile, as it could be used in declarations as
well.  Making it type-safe would require the use of typeof_unqual from
C23, though.

So use the safe and compatible variant for now.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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