reftable: consistently refer to `reftable_write_options` as `opts`

reftable: consistently refer to `reftable_write_options` as `opts`

Throughout the reftable library the `reftable_write_options` are
sometimes referred to as `cfg` and sometimes as `opts`. Unify these to
consistently use `opts` to avoid confusion.

While at it, touch up the coding style a bit by removing unneeded braces
around one-line statements and newlines between variable declarations.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>