refs: clear errno return in refs_resolve_ref_unsafe()

refs: clear errno return in refs_resolve_ref_unsafe()

This is done in a separate commit, to pinpoint the precise cause should there be
regressions in error reporting.

This is implemented by renaming the existing logic to a static function
refs_resolve_unsafe_implicit_errno(), minimizing the code diff.

Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Reviewed-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>