bug#8874: Redisplay confusion in Info, due to recentering fix

[Chong Yidong]

Recipe to reproduce:

emacs -q
M-: (setq scroll-conservatively 300) RET
C-h i
m CC mode RET
C-s k&r
C-s C-s C-s C-s

Emacs redisplay now shows a garbage window with no cursor.  C-l redraws
the window correctly.

This is caused by the recentering changes from March (revision 103792).
Reverting it eliminates the bug.