Scroll forward, reverting buffer if needed, in View mode.
If buffer has not been changed and the corresponding file is newer, first
revert the buffer, then scroll.
This command is useful if you are viewing a changing file.
The prefix argument LINES says how many lines to scroll.
If you don't specify a prefix argument, it uses the number of lines set by
M-x View-scroll-page-forward-set-page-size or
M-x View-scroll-page-backward-set-page-size.
If LINES is more than a window-full, only the last window-full is shown.