consider clustered primary keys

Choosing a shortest index for the covering index scan,
the optimizer ignored the fact, that the clustered primary
key read involves whole table data.

The find_shortest_key function has been modified to
take into account that fact that a clustered PK has a
longest key of possible covering indices.


mysql-test/r/innodb_mysql.result:
  Test case for bug #39653.
mysql-test/t/innodb_mysql.test:
  Test case for bug #39653.
sql/sql_select.cc:
  Bug #39653: find_shortest_key in sql_select.cc does not
              consider clustered primary keys
  
  The find_shortest_key function has been modified to
  take into account that fact that a clustered PK has a
  longest key of possible covering indices.