Set a flag when a SHOW command is parsed, and check it in log_slow_statement().  SHOW commands are not counted as slow queries, even if they use table scans.


mysql-test/t/show_check-master.opt:
  BitKeeper file /usr/home/tim/m/bk/b19764/50/mysql-test/t/show_check-master.opt
mysql-test/r/show_check.result:
  Add test for bug #19764
mysql-test/t/show_check.test:
  Add test for bug #19764
sql/sql_lex.cc:
  Set lex->is_show_command= FALSE in lex_start
sql/sql_lex.h:
  Add LEX->is_slow_command flag to prevent SHOW commands from being written to the slow queries log
sql/sql_parse.cc:
  Don't log slow statement if it is a SHOW command
sql/sql_yacc.yy:
  Set lex->is_show_command for all SHOW commands