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>

refs/ref-cache.h

@@ -42,6 +42,7 @@ struct ref_value {
 	 * referred to by the last reference in the symlink chain.
 	 */
 	struct object_id oid;
+	char *referent;
 };
 
 /*
@@ -173,6 +174,7 @@ struct ref_entry *create_dir_entry(struct ref_cache *cache,
 				   const char *dirname, size_t len);
 
 struct ref_entry *create_ref_entry(const char *refname,
+				   const char *referent,
 				   const struct object_id *oid, int flag);
 
 /*