When mysqldump tries to dump information in xml format,
the result does not contain field level comments.

In order to retrieve various informations for a field/column,
mysqldump currently uses 'show fields from <tab>' statement.
The attributes returned by the statement lacks the information
regarding field comments.

Fixed by changing the query to one that probes I_S to retrieve
required field informations, including the field comment.


client/mysqldump.c:
  Bug#13618 : mysqldump --xml omits comment on table field.
  
  Replaced the 'show fields' command by a statement that
  queries I_S, in order to retrieve information on all the
  attributes that 'show fields' returns along-with an additional
  column_comment information.
mysql-test/r/client_xml.result:
  Result modifications for bug#13618.
mysql-test/r/mysqldump.result:
  Result modifications for bug#13618.
mysql-test/t/mysqldump.test:
  Added a testcase for bug#13618.