builtin/show-ref: ensure mutual exclusiveness of subcommands

builtin/show-ref: ensure mutual exclusiveness of subcommands

The git-show-ref(1) command has three different modes, of which one is
implicit and the other two can be chosen explicitly by passing a flag.
But while these modes are standalone and cause us to execute completely
separate code paths, we gladly accept the case where a user asks for
both `--exclude-existing` and `--verify` at the same time even though it
is not obvious what will happen. Spoiler: we ignore `--verify` and
execute the `--exclude-existing` mode.

Let's explicitly detect this invalid usage and die in case both modes
were requested.

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