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

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

The static add_head_info() function is only used indirectly by callers
of get_worktrees(), none of whom care about errno, and even if they
did having the faked-up one from refs_resolve_ref_unsafe() would only
confuse them if they used die_errno() et al. So let's explicitly
ignore it here.

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