Error thrown when there're two table with same name but from different database in the UPDATE stmt

[winoros]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.

use test;
create table t(a int);
create database test2;
create test2.t(a int);
update test.t, test2.t set test.t.a=test.t.a+100;

2. What did you expect to see?

Executes successfully.

3. What did you see instead?

Error ambiguous column returned.

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?