refs: keep track of unresolved reference value in iterators

refs: keep track of unresolved reference value in iterators

Since ref iterators do not hold onto the direct value of a reference
without resolving it, the only way to get ahold of a direct value of a
symbolic ref is to make a separate call to refs_read_symbolic_ref.

To make accessing the direct value of a symbolic ref more efficient,
let's save the direct value of the ref in the iterators for both the
files backend and the reftable backend.

Signed-off-by: John Cai <johncai86@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>