builtin/show-ref: stop using global vars for `show_one()`

builtin/show-ref: stop using global vars for `show_one()`

The `show_one()` function implicitly receives a bunch of options which
are tracked via global variables. This makes it hard to see which
subcommands of git-show-ref(1) actually make use of these options.

Introduce a `show_one_options` structure that gets passed down to this
function. This allows us to get rid of more global state and makes it
more explicit which subcommands use those options.

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