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

[Charles A. Roelli]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Date: Wed, 13 Mar 2019 15:56:50 -0400
> 
> >> There are cases where recentering does "make sure that the whole entry
> >> is visible", but as we saw in the first post of this report, there is
> >> at least one case where recentering makes less useful context visible.
> > That's clearly undesirable, indeed.  I think it's a plain bug.
> > Basically when moving in one direction, diff-hunk-next/prev should never
> > scroll the buffer in the other direction.
> 
> I installed the patch below which should fix this most glaring problem.
> 
> 
>         Stefan

Thanks!  I noticed that I had to "touch lisp/vc/diff-mode.el" before
the change took effect in my local tree, since the change in the macro
does not trigger the recompilation of the callers.  Could "make"
somehow detect this?