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

[ndame]

>
> That's also been discussed extensively during the M-x man wars, and it's
> a horrible UX -- having windows popping up asynchronously at some later
> point is disruptive and confusing, so we don't do that.
>

Diff output arrives pretty much instantly, the user does not even lift his
fingers from executing the diff key binding when the beginning of the output is
already there.

And the user expects something to happen when invoking diff, either seeing the
diff or getting the message there is no diff, so waiting for a fraction of a 
second
before popping up a window is not a real problem, because the user is waiting 
for the
result anyway.