builtin/show-ref: refactor `--exclude-existing` options

builtin/show-ref: refactor `--exclude-existing` options

It's not immediately obvious options which options are applicable to
what subcommand in git-show-ref(1) because all options exist as global
state. This can easily cause confusion for the reader.

Refactor options for the `--exclude-existing` subcommand to be contained
in a separate structure. This structure is stored on the stack and
passed down as required. Consequently, it clearly delimits the scope of
those options and requires the reader to worry less about global state.

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