Change scoring strategy for 'flex' completion style

Change scoring strategy for 'flex' completion style

The previous strategy had problems comparing scores of matches to
strings of different lengths.  This one seems slightly more sensible,
and uses a new constant `flex-score-match-tightness' instead of the
more abstract `flex-score-falloff'.

It's not completely without problems, and I think it shouldn't count
"holes" at the front and at the back, but that needs a different
"pattern-to-regexp" conversion in completion-pcm--hilit-commonality.

(defun test ()
  (mapcar (lambda (a)
            (cons (substring-no-properties a)
                  (get-text-property 0 'completion-score a)))
          (sort (completion-pcm--hilit-commonality
                  '(prefix "f" star "o" star "o" point)
                  '("foo"
                    "barfoobaz"
                    "foobarbaz"
                    "barbazfoo"
                    "fabrobazo"
                    "foot"
                    "foto"
                    "fotttttttttttttttttttttttto"))
      (lambda (a b)
        (> (get-text-property 0 'completion-score a)
           (get-text-property 0 'completion-score b))))))

(let ((flex-score-match-tightness 100)) (test))
=> (("foo" . 1.0)
    ("foot" . 0.375)
    ("foto" . 0.375)
    ("foobarbaz" . 0.16260162601626016) ;; one hole
    ("barbazfoo" . 0.16260162601626016) ;; one hole
    ("barfoobaz" . 0.10964912280701755) ;; two holes
    ("fabrobazo" . 0.10964912280701755) ;; two holes
    ("fotttttttttttttttttttttttto" . 0.04982561036372696))

(let ((flex-score-match-tightness 0.1)) (test))
=> (("foo" . 1.0)
    ("foot" . 0.375)
    ("foto" . 0.375)
    ("barfoobaz" . 0.007751937984496124) ;; two holes
    ("fabrobazo" . 0.007751937984496124) ;; two holes
    ("foobarbaz" . 0.00641025641025641)  ;; one hole
    ("barbazfoo" . 0.00641025641025641)  ;; one hole
    ("fotttttttttttttttttttttttto" . 0.0004789272030651341))

* lisp/minibuffer.el (flex-score-falloff): Rename to
flex-score-match-tightness.
(completion-pcm--hilit-commonality): Update function.