t: harmonize t-reftable-merged.c with coding guidelines

t: harmonize t-reftable-merged.c with coding guidelines

Harmonize the newly ported test unit-tests/t-reftable-merged.c
with the following guidelines:
- Single line control flow statements like 'for' and 'if'
  must omit curly braces.
- Structs must be 0-initialized with '= { 0 }' instead of '= { NULL }'.
- Array indices should preferably be of type 'size_t', not 'int'.
- It is fine to use C99 initial declaration in 'for' loop.

While at it, use 'ARRAY_SIZE(x)' to store the number of elements
in an array instead of hardcoding them.

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>