when index is used

When the table contained TEXT columns with empty contents
('', zero length, but not NULL) _and_ strings starting with
control characters like tabulator or newline, the empty values
were not found in a "records in range" estimate. Hence count(*)
missed these records.

The reason was a different set of search flags used for key
insert and key range estimation.

I decided to fix the set of flags used in range estimation.
Otherwise millions of databases around the world would require
a repair after an upgrade.

The consequence is that the manual must be fixed, which claims
that TEXT columns are compared with "end space padding". This
is true for CHAR/VARCHAR but wrong for TEXT. See also bug 21335.


myisam/mi_range.c:
  Bug#26231 - select count(*) on myisam table returns wrong value
              when index is used
  Added SEARCH_UPDATE to the search flags so that it compares
  like write/update/delete operations do. Only so it expects
  the keys at the place where they have been inserted.
myisam/mi_search.c:
  Bug#26231 - select count(*) on myisam table returns wrong value
              when index is used
  Added some comments to explain how _mi_get_binary_pack_key()
  works.
mysql-test/r/myisam.result:
  Bug#26231 - select count(*) on myisam table returns wrong value
              when index is used
  Added a test.
mysql-test/t/myisam.test:
  Bug#26231 - select count(*) on myisam table returns wrong value
              when index is used
  Added test result.