bug#34723: 27.0.50; customize and improve diff-mode recentering

[Charles A. Roelli]

> Date: Tue, 05 Mar 2019 21:44:01 +0200
> From: Eli Zaretskii <eliz@gnu.org>
> CC: 34723@debbugs.gnu.org
> 
> I don't understand what calling 'recenter' has to do with
> scroll-conservatively.  And scroll-conservatively is implemented in
> the display engine, so I'm afraid I'm still in the dark regarding the
> nature of the problem you are discussing.

Does the following scenario make some sense?  diff-hunk-next moves
point to a position beyond (window-end), and immediately calls
(recenter '(0)).  Scrolling (which would otherwise respect
scroll-conservatively) gets preempted by diff-hunk-next's call to
recenter.

> Of course, if you know what to do to fix the problem, there's no need
> for me to understand the issue, and I should just shut up and let you
> do what you think is right.

(Please don't, I will almost certainly screw it up. ;-)