bug#12170: save-excursion fails boundary case with recenter

[Stefan Monnier]

> But I don't have the slightest idea how calling
>        (set-window-start (selected-window) ,start t)
> would remedy this.  Eli will soon teach us a lesson here.

The redisplay tries to make sure (window-)point is visible, which can
either be done by changing (window-)point or by changing window-start.

The first happens when you use a scrolling operation (which sets
window-start), the other happens in response to normal navigation in
the buffer.

Knowing which to do when is not obvious, but IIUC if window-start has
not been changed, then the redisplay assumes that it is free to change
(window-)point, whereas otherwise it will assume that it should not
change (window-)point but can instead change window-start.


        Stefan