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

[Eli Zaretskii]

> Date: Sat, 11 Aug 2012 18:05:14 +0200
> From: martin rudalics <rudalics@gmx.at>
> CC: wbrodie@panix.com, 12170@debbugs.gnu.org
> 
> So the difference should be in one of the four assignments
> 
>    w->start_at_line_beg = 0;
>    w->update_mode_line = 1;
>    w->last_modified = 0;
>    w->last_overlay_modified = 0;
> 
> and I'd obviously vote for the first one.  Interesting side-effect.

FWIW, there are only 2 places in redisplay that look at
w->start_at_line_beg, but neither your nor Bill's recipe don't
exercise the code of either of these two places.