refs API: ignore errno in worktree.c's find_shared_symref()

refs API: ignore errno in worktree.c's find_shared_symref()

There are only handful of callers of find_shared_symref(), none of
whom care about errno, so let's migrate to the non-errno-propagating
version of refs_resolve_ref_unsafe() and explicitly ignore errno here.

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