mysql-test/r/select.result:
  Test for BUG#12977.
mysql-test/t/select.test:
  Test for BUG#12977.
sql/sql_base.cc:
  - Compare table qualifier of qualified fields only with tables that
    are not natural joins or their operands.
  - For qualified fields perform recursive search in all operands of
    natural joins that are nested joins.
  - Symmetrically detect ambiguous columns for both operands of
    NATURAL/USING joins.