Merge branch 'jc/refs-symref-referent'

Merge branch 'jc/refs-symref-referent'

The refs API has been taught to give symref target information to
the users of ref iterators, allowing for-each-ref and friends to
avoid an extra ref_resolve_* API call per a symbolic ref.

* jc/refs-symref-referent:
  ref-filter: populate symref from iterator
  refs: add referent to each_ref_fn
  refs: keep track of unresolved reference value in iterators