Robustify completion match scoring for optimized patterns

Robustify completion match scoring for optimized patterns

Fixes: bug#42149
The function completion-pcm--hilit-commonality, which propertizes and
scores a previously confirmed match, expected its PATTERN argument to
match the strings of COMPLETIONS entirely (i.e. up to the string's
very end).  But sometimes the ending wildcard, represented by the
'any' atom in PATTERN, is optimized away by
completion-pcm--optimize-pattern.  Although this is mostly benign in
terms of highlighting commonality, it leads to incorrect score values.

In this change, we ensure that completion-pcm--hilit-commonality is
aware of this exception and isn't affected by it.  We also document
the function a bit better and simplify its workings.

Originally reported by Dario Gjorgjevski <dario.gjorgjevski@gmail.com>

* lisp/minibuffer.el (completion-pcm--hilit-commonality):
Simplify.  Add docstring.

* lisp/minibuffer.el (completion-pcm--hilit-commonality): Add
docstring