Improve scoring algorithm for flex-style completions

Improve scoring algorithm for flex-style completions

The previous algorithm had two problems: it considered non-matches in
the beginning and end of the string as matching "holes" and failed to
penalize larger holes, making flex-score-match-tightness only
effective in some corner cases.

The new formula, which is described in code and in pseudo-code in the
comments, fixes these problems.

As a result, by default, C-h f flex now correctly bubbles up
"company-search-flex-regexp" to the top, in front of "file-exists-p".
With a flex-score-match-tightness smaller than 1.0, the situation is
reversed.

* lisp/minibuffer.el (flex-score-match-tightness): Adjust default
value.  Improve docstring example.
(completion-pcm--hilit-commonality): Improve example.  Remove unused
variable.  Improve algorithm.