The reftable library provides the ability to swap out allocators. There
is a gap here though, because we continue to use `xstrdup()` even in the
case where all the other allocators have been swapped out.
Introduce `reftable_strdup()` that uses `reftable_malloc()` to do the
allocation.
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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