Disallow '-' as tag name, as well as tag names starting with '-', as it
would be cumbersome to "git checkout tags/-" because "git checkout -" is
to switch to the previous branch.
Add strbuf_check_tag_ref() as helper to check a refname for a tag.
Signed-off-by: Michael Schubert <mschub@elegosoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>