Bug #30596 GROUP BY optimization gives wrong result order

The optimization that uses a unique index to remove GROUP BY, did not 
ensure that the index was actually used, thus violating the ORDER BY
that is impled by GROUP BY.
Fixed by replacing GROUP BY with ORDER BY if the GROUP BY clause contains
a unique index. In case GROUP BY ... ORDER BY null is used, GROUP BY is
simply removed.


BitKeeper/etc/ignore:
  Added support-files/mysqld_multi.server tests/bug25714 cscope.in.out cscope.out cscope.po.out to the ignore list
mysql-test/r/distinct.result:
  Bug#30596: Changed test case. 
  Prior to Bug#16458, These queries use temp table and filesort. The
  bug was that they used a temp table. However, that patch removed
  filesort also, in which case we can no longer gurantee correct ordering.
mysql-test/r/group_by.result:
  Bug#30596: Correct result
mysql-test/r/innodb_mysql.result:
  Bug#30596: Test case for innodb. Here, as opposed to for MyISAM, row 
  lookup is done using index whenever the index covers the group list.
mysql-test/t/group_by.test:
  Bug#30596: Test case
mysql-test/t/innodb_mysql.test:
  Bug#30596: Test case
sql/sql_select.cc:
  Bug#30596: The fix, replacing GROUP BY with ORDER BY unless 
  ORDER BY [NULL|<constant>]

.bzrignore

@@ -1345,3 +1345,8 @@ zlib/*.vcproj
 debian/control
 debian/defs.mk
 include/abi_check
+support-files/mysqld_multi.server
+tests/bug25714
+cscope.in.out
+cscope.out
+cscope.po.out