bug#14708: 24.2; query-replace-regexp when match and replacement are the same

[Ed Avis]

I wouldn't say that the regexp in this case is broken - only suboptimal. The 
user entered something correct, to replace all sequences of spaces by a single 
space. If done as a global replace without confirmation, you wouldn't even 
notice. The user asked for confirmation of each change to end up with the right 
text in the buffer, not as an academic exercise in finding the best possible 
regexp.

So prompting for each no-op match is not really helping the user. I can see 
that sometimes you might use query-replace-regexp as a debugging aid (though 
surely a simple search without replacement would be better). But even then it 
would be enough just to flag up one case where match=replacement, not request y 
or n for each one, since they are all identical. (Here I am assuming no 
capturing groups.)

So maybe the answer is to flag the first no-op match but then skip the rest. 

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