bug#49774: 27.1; if diff commands finds no differences then do not pop up a window

[ndame]

>
> > Diff output arrives pretty much instantly
>
> Depends on many things -- the files may be on remote systems, for
> instance.

Okay, then how about adding a configurable delay before popping up the diff 
window?
E.g. if no results arrive within, say, 1 second then a timer pops up the
window, so the user knows the diff is in progress, but slow.

This takes care of the slow remote system problem and the timer would 
practically
never  fire on a local machine where the filesystem response is quick.