According to the readline library documentation, the callback passed to
rl_callback_handler_install is supposed to free the string passed to it:

> As with readline(), the handler function should free the line when it
> it finished with it.

( See https://tiswww.case.edu/php/chet/readline/readline.html#SEC41 )

Our callback leaks it.

Closes #6817

NO_DOC=bug fix