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.

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