refs/reftable: refactor reading symbolic refs to use reftable backend

Refactor the callback function that reads symbolic references in the
reftable backend to use `reftable_backend_read_ref()` instead of
accessing the reftable stack directly. This ensures that the function
will benefit from the new caching layer that we're about to introduce.

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

refs/reftable-backend.c

@@ -884,21 +884,18 @@ static int reftable_be_read_symbolic_ref(struct ref_store *ref_store,
 {
 	struct reftable_ref_store *refs =
 		reftable_be_downcast(ref_store, REF_STORE_READ, "read_symbolic_ref");
-	struct reftable_ref_record ref = {0};
 	struct reftable_backend *be;
+	struct object_id oid;
+	unsigned int type = 0;
 	int ret;
 
 	ret = backend_for(&be, refs, refname, &refname, 1);
 	if (ret)
 		return ret;
 
-	ret = reftable_stack_read_ref(be->stack, refname, &ref);
-	if (ret == 0 && ref.value_type == REFTABLE_REF_SYMREF)
-		strbuf_addstr(referent, ref.value.symref);
-	else
+	ret = reftable_backend_read_ref(be, refname, &oid, referent, &type);
+	if (type != REF_ISSYMREF)
 		ret = -1;
-
-	reftable_ref_record_release(&ref);
 	return ret;
 }