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

[Charles A. Roelli]

> Date: Tue, 05 Mar 2019 22:22:41 +0200
> From: Eli Zaretskii <eliz@gnu.org>
> 
> It keeps point closer to the center of the window and away from the
> window edges.  I though that's what you wanted, because doing that
> will show more context around the match.

Right.  I spoke earlier about calling reposition-window from
diff-hunk-next as a possible replacement for recenter, and Juri
suggested an equivalent behavior for Isearch, and the aim of these
changes is to show the most useful context around point -- which does
not necessarily mean keeping point close to the center of the window.
For example, with the Isearch example I gave, point can end up
anywhere in the window after searching, since reposition-window tries
to position the first line of the function definition on the first
screen line.