Reworked 2001-06-01 change.

Reworked 2001-06-01 change.
(choose-completion-string1): Merged back into choose-completion-string.
(choose-completion-string): Run choose-completion-string-functions
after checking for proper minibuffer window.  Added mini-p arg to
the hook functions.  Insert choice if all hook functions return nil.
(choose-completion-string-functions): Doc update.