t-reftable-readwrite: use free_names() instead of a for loop

t-reftable-readwrite: use free_names() instead of a for loop

free_names() as defined by reftable/basics.{c,h} frees a NULL
terminated array of malloced strings along with the array itself.
Use this function instead of a for loop to free such an array.

Mentored-by: Patrick Steinhardt <ps@pks.im>
Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Chandra Pratap <chandrapratap3519@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>