bug#25260: 25.1; cd change global variables permanently

[Leo Liu]

As a fix to https://debbugs.gnu.org/7924 `cd' changes
minibuffer-completion-table and minibuffer-completion-predicate
globally. That looks like a mistake. Stefan, did you intend to write
setq-local instead?

I only discovered this because icomplete installs a post-command-hook if
minibuffer-completion-table is non-nil which has been annoying me for
two days.

Happy holidays,
Leo