bug#23399: 25.0.93; follow-scroll-up not working properly

[Milan Zamazal]

In GNU Emacs 25.0.93.2 (x86_64-unknown-linux-gnu, GTK+ Version 2.24.29)
 of 2016-04-27 built on pdm

follow-scroll-up works the same way as scroll-up instead of scrolling to
the bottom of the last window in the follow chain.

How to reproduce the bug:

- Run Emacs: emacs -Q
- Type `C-h t' to display the tutorial.
- Type `C-x 3' to split the window.
- Type `M-x follow-mode RET' to switch to follow-mode.
- Type `C-c . C-v' to invoke follow-scroll-up.

The buffer is scrolled up only to the bottom of the left window (to
"From now on, ...") instead to the bottom of the right window (to
"\n>> Do a few C-n's ..."), which is a bug and regression from
Emacs 24.5.

There is the same bug with follow-scroll-down.