refs API: make resolve_gitlink_ref() not set errno

refs API: make resolve_gitlink_ref() not set errno

I have carefully read the upstream callers of resolve_gitlink_ref()
and determined that they don't care about errno.

So let's move away from the errno-setting refs_resolve_ref_unsafe()
wrapper to refs_werrres_ref_unsafe(), and explicitly ignore the errno
it sets for us.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>