reftable: remove unnecessary curly braces in reftable/tree.c
According to Documentation/CodingGuidelines, single-line control-flow
statements must omit curly braces (except for some special cases).
Make reftable/tree.c adhere to this guideline.
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>