Rework face hints for partial-string completion styles

Rework face hints for partial-string completion styles

Don't use completions-first-difference for the 'substring', 'flex' and
'partial-completion' styles, since there can be really no reasonable
definition of a "first" difference there.

Make completions-common-part inherit from 'underline' so that it is
useful by default for all completion styles.

* lisp/minibuffer.el (completions-common-part): Adjust
description and change default value.
(completions-first-difference): Ajust docstring.
(completion-pcm--hilit-commonality): Don't use
completions-first-difference.