Keep pre-existing highlighting in completion candidates.

Keep pre-existing highlighting in completion candidates.
* lisp/minibuffer.el (completions-first-difference): State that the
face is "added" in the docstring.
(completions-common-part): Same.  And don't inherit from default.
(completion-hilit-commonality): Prepend 'completions-common-part
and 'completion-first-difference faces to the 'face property,
instead of replacing the value(s).
(completion--insert-strings): Same with 'completions-annotations face.
(completion-hilit-commonality): Use 'face instead of
'font-lock-face, because it gets priority if the completion
strings already have 'face set.