bug#46783: Catch mistaken (query-replace "a" "a")

[積丹尼 Dan Jacobson]

(query-replace "a" "a") should ask "Are you sure?"
Better yet: "Are you nuts?"...

So how could it happen?

Well, e.g.,
the user needs to replace some character with a similar character, all
through the document. So similar in fact that it is hard to tell the
difference.

Therefore emacs should be a good Samaritan and please "Refuse loudly to
do one big no-op that will make the user furious once he discovers he
meticulously did... *nothing at all*."

Imagine answering
ynnyynnnnnnyynynnnynyyyyynynyyyyyynnnnynnnynynynyny only to find out it was 
just a big
nnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn !

So emacs should simply double check the strings before starting.

No the user can't always tell, no matter how hard he looks, if they are
the same or not.

One might say "well there are still some uses for replacing the same
thing with itself." No, wrong.