ref-filter: make ref_array_item allocation more consistent

ref-filter: make ref_array_item allocation more consistent

We have a helper function to allocate ref_array_item
structs, but it only takes a subset of the possible fields
in the struct as initializers. We could have it accept an
argument for _every_ field, but that becomes a pain for the
fields which some callers don't want to set initially.

Instead, let's be explicit that it takes only the minimum
required to create the ref, and that callers should then
fill in the rest themselves.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>