ref-filter: populate symref from iterator

ref-filter: populate symref from iterator

With a previous commit, the reference the symbolic ref points to is saved
in the ref iterator records. Instead of making a separate call to
resolve_refdup() each time, we can just populate the ref_array_item with
the value from the iterator.

Signed-off-by: John Cai <redacted>
Signed-off-by: Junio C Hamano <redacted>