[patch] : Fix std::mismatch concept check

[Paolo Carlini]

Hi,

> This patch fixes the concept check on std::mismatch. Given 
> mismatch(first1, last1, first2), the old concept check made sure that 
> *first1==*first1 and *first2==*first2 are defined. In fact what is 
> required is that *first1=*first2 is required.

your patch seems obviously correct to me (indeed, std::equal below is 
already doing the right thing) and, if nobody objects, I gonna apply it 
first.

Thanks!
Paolo.