reftable/record: stop using `COPY_ARRAY()`

reftable/record: stop using `COPY_ARRAY()`

Drop our use of `COPY_ARRAY()`, replacing it with an open-coded variant
thereof. This is done to reduce our dependency on the Git library.

While at it, guard the whole array copy logic so that we only copy it in
case there actually is anything to be copied. Otherwise, we may end up
trying to allocate a zero-sized array, which will return a NULL pointer
and thus cause us to return an `REFTABLE_OUT_OF_MEMORY_ERROR`.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>