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

[Juri Linkov]

> Can we add something like this to isearch.el, maybe as a defcustom?
> Or maybe we could add it in a way such that other packages that show
> search results (grep, xref) could use the same function to reposition
> the window.  The function could inspect a defcustom in simple.el to
> decide whether (or how) to reposition the window.

Yes, a more common customization would be preferable.
Something at the same low level as `recenter',
maybe adding a new defcustom like `recenter-function'
to run a customized function e.g. `reposition-window'
instead of the default recentering to the middle of the screen.