t-reftable-readwrite: use 'for' in place of infinite 'while' loops

t-reftable-readwrite: use 'for' in place of infinite 'while' loops

Using a for loop with an empty conditional statement is more concise
and easier to read than an infinite 'while' loop in instances
where we need a loop variable. Hence, replace such instances of a
'while' loop with the equivalent 'for' loop.

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>