* lisp/minibuffer.el: Tweak and undo parts of recent changes

* lisp/minibuffer.el: Tweak and undo parts of recent changes

(completion-metadata): Always return a fresh new cons cell.
(completion--nth-completion): Don't bother calling adjust-metadata
if the result won't be used.
(completion-pcm--hilit-commonality): Revert recent change which had
removed support for `completions-first-difference` in `substring` and
`partial-completion` styles.
(completion--flex-adjust-metadata): Treat the arg as immutable.