bug#19259: 25.0.50; Be able to use `compare-windows' in either direction

[Drew Adams]

> > Especially when `compare-windows' is used repeatedly (e.g. bound
> > to a repeatable key etc.), it can be helpful to be able to
> > sometimes compare backward, toward bob, instead of always forward,
> > toward eob.  Please add this enhancement.
> 
> Why do you want this feature?  After comparing from the top of the
> buffer you can return to previous matches by popping previous
> positions from the mark ring with `C-u C-SPC C-SPC C-SPC ...'

It's not just about returning to positions.  And you don't necessarily
want to pop, just to navigate.  And show the highlighting.

And you don't necessarily want to start at bob.  (But you might want
to wrap around to there - that is not part of this enhancement request).

Think ediff.  With ediff you can easily go back and forth, either
direction, from any difference position, restoring its highlighting.

You can even jump to a particular difference (`j').  And show the
difference/comparison at point (`g').  The idea is to make
`compare-windows' similarly useful.