[Bug libstdc++/57916] Improve std::sort paritioning by explicitly employing the pivot

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57916

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Thanks for the patch, do you have a GCC opyright assignment in place?

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