This patch adds cost estimation for the queries with ORDER BY / GROUP BY
and LIMIT. 
If there was a ref/range access to the table whose rows were required
to be ordered in the result set the optimizer always employed this access
though a scan by a different index that was compatible with the required 
order could be cheaper to produce the first L rows of the result set.
Now for such queries the optimizer makes a choice between the cheapest
ref/range accesses not compatible with the given order and index scans
compatible with it.


mysql-test/r/distinct.result:
  Adjusted results for test cases affected fy the fix for bug #28404.
mysql-test/r/endspace.result:
  Adjusted results for test cases affected fy the fix for bug #28404.
mysql-test/r/group_by.result:
  Adjusted results for test cases affected fy the fix for bug #28404.
mysql-test/r/group_min_max.result:
  Adjusted results for test cases affected fy the fix for bug #28404.
mysql-test/r/innodb.result:
  Adjusted results for test cases affected fy the fix for bug #28404.
mysql-test/r/innodb_mysql.result:
  Adjusted results for test cases affected fy the fix for bug #28404.
mysql-test/r/merge.result:
  Adjusted results for test cases affected fy the fix for bug #28404.
mysql-test/r/order_by.result:
  Added a test case for bug #28404.
mysql-test/r/select_found.result:
  Adjusted results for test cases affected fy the fix for bug #28404.
mysql-test/r/subselect.result:
  Adjusted results for test cases affected fy the fix for bug #28404.
mysql-test/t/distinct.test:
  Changed a test case after adding the fix for bug #28404.
mysql-test/t/order_by.test:
  Added a test case for bug #28404.
sql/sql_select.cc:
  Fixed bug#28404.
  This patch adds cost estimation for the queries with ORDER BY / GROUP BY
  and LIMIT. 
  Now for such queries the optimizer makes a choice between the cheapest
  ref/range accesses not compatible with the given order and index scans
  compatible with it.
  
  Modified the function test_if_skip_sort_order to make the above mentioned
  choice cost based.
sql/sql_select.h:
  Fixed bug#28404.
  This patch adds cost estimation for the queries with ORDER BY / GROUP BY
  and LIMIT. 
  Added a new field fot the JOIN_TAB structure.