builtin/show-ref: stop using global variable to count matches

builtin/show-ref: stop using global variable to count matches

When passing patterns to git-show-ref(1) we're checking whether any
reference matches -- if none do, we indicate this condition via an
unsuccessful exit code.

We're using a global variable to count these matches, which is required
because the counter is getting incremented in a callback function. But
now that we have the `struct show_ref_data` in place, we can get rid of
the global variable and put the counter in there instead.

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