bug#21893: 25.0.50; Using scroll-*-command in follow-mode

[Juri Linkov]

follow.el could use better commands scroll-up-command/scroll-down-command
like in this patch to work smoothly when scroll-error-top-bottom is t,
but a special condition in follow-scroll-up/follow-scroll-down
that checks for scroll-preserve-screen-position prevents from scrolling
and returns point to the old position, and there is no comment
explaining this special-casing.

diff --git a/lisp/follow.el b/lisp/follow.el
index 938c59e..fa24a28 100644
--- a/lisp/follow.el
+++ b/lisp/follow.el
@@ -518,14 +518,14 @@ (defun follow-scroll-up (&optional arg)
 Works like `scroll-up' when not in Follow mode."
   (interactive "P")
   (cond ((not follow-mode)
-        (scroll-up arg))
+        (scroll-up-command arg))
        ((eq arg '-)
         (follow-scroll-down))
        (t
         (let ((opoint (point))  (owin (selected-window)))
           (while
               ;; If we are too near EOB, try scrolling the previous window.
-              (condition-case nil (progn (scroll-up arg) nil)
+              (condition-case nil (progn (scroll-up-command arg) nil)
                 (end-of-buffer
                  (condition-case nil (progn (follow-previous-window) t)
                    (error
@@ -551,12 +551,12 @@ (defun follow-scroll-down (&optional arg)
 Works like `scroll-down' when not in Follow mode."
   (interactive "P")
   (cond ((not follow-mode)
-        (scroll-down arg))
+        (scroll-down-command arg))
        ((eq arg '-)
         (follow-scroll-up))
        (t
         (let ((opoint (point)))
-          (scroll-down arg)
+          (scroll-down-command arg)
           (unless (and scroll-preserve-screen-position
                        (get this-command 'scroll-command))
             (goto-char opoint))