bug#57669: 29.0.50; C-n, C-p off under long lines

[dick <dick.r.chiang@gmail.com>]

> Try the same with a buffer with 10 lines, each of which is 10 MB long.

I don't have the patience for 10MB.  How about 0.1MB?

OUTPUT:

Under long-line-threshold=nil find-file ineffable
Under long-line-threshold=10000 find-file 99s
Under long-line-threshold=nil find-file-literally ineffable
Under long-line-threshold=10000 find-file-literally 8s

INPUT:

src/emacs -Q -l cl-lib --eval "                                   \
(let ((large-file-warning-threshold (truncate 1e9))               \
      (long-line-threshold 10000)                                 \
      (file (make-temp-file \"baz\")))                            \
  (cl-assert (= 80 (window-width)) t)                             \
  (with-temp-file file						  \
    (dotimes (i 10)                                               \
      (insert (make-string (truncate 1e5) ?x) \"\n\")))           \
  (dolist (config (list (cons nil (function find-file))           \
                        (cons long-line-threshold (function find-file))         \
                        (cons nil (function find-file-literally)) \
                        (cons long-line-threshold (function find-file-literally)))) \
    (let* ((long-line-threshold (car config))                     \
           (note (format \"long-line-threshold=%s %s\"            \
                         (car config) (cdr config)))              \
           (buffer (funcall (cdr config) file)))                  \
        (if (null (car config))                                   \
          (princ (format \"Under %s ineffable\n\" note)           \
                (function external-debugging-output))             \
        (with-current-buffer buffer                               \
          (show-paren-mode -1)                                    \
          (goto-char (point-min))                                 \
          (princ (format \"Under %s %ds\n\" note                  \
                    (fround (car (benchmark-run                   \
                         (while (condition-case nil               \
                             (prog1 t (call-interactively         \
                               (function next-line))              \
                             (redisplay))                         \
                           (end-of-buffer)))))))                  \
            (function external-debugging-output)))                \
        (kill-buffer buffer))))                                   \
   (kill-emacs))"