After dropping and recreating the database specified along with --one-database
option at command line, mysql client keeps filtering the statements even after
the execution of a 'USE' command on the same database.

--one-database option enables the filtering of statements when the current
database is not the one specified at the command line. However, when the same
database is dropped and recreated the variable (current_db) that holds the
inital database name gets altered. This bug exploits the fact that current_db
initially gets set to null value (0) when a 'use db_name' follows the recreation
of same database db_name (speficied at the command line) and hence skip_updates
gets set to 1, which inturn triggers the further filtering of statements.

Fixed by making get_current_db() a no-op function when one_database is set,
and hence, under that condition current_db will not get altered.
Note, however the value of current_db can change when we execute 'connect'
command with a differnet database to reconnect to the server, in which case,
the behavior of --one-database will be formulated using this new database.


client/mysql.cc:
  Bug #54899 : --one-database option cannot handle DROP/CREATE DATABASE commands
  
  Added an if statement at the beginnning of get_current_db() , which makes it
  a no-op function if one-database option is specified, and hence current_db
  remains unchanged.
  
  Changed the help message for one-database option to a more appropriate message
  as specified in mysql documentation.
mysql-test/r/mysql.result:
  Added a test case for bug#54899 and some more test cases to
  check other one-database option related behaviors.
mysql-test/t/mysql.test:
  Added a test case for bug#54899 and some more test cases to
  check other one-database option related behaviors.