do full type check in BARF_UNLESS_COPYABLE

do full type check in BARF_UNLESS_COPYABLE

Use __builtin_types_compatible_p to perform a full type check if
possible.  Otherwise fall back to the old size comparison, but add a
non-evaluated assignment to catch more type mismatches.  It doesn't flag
copies between arrays with different signedness, but that's as close to
a full type check as it gets without the builtin, as far as I can see.

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