reftable: introduce `reftable_strdup()`

reftable: introduce `reftable_strdup()`

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>