BUG: Fix mergesort unstable when ascending=False (GH6399)
closes #6399. I hope I followed all of the correct procedures.

I ran the full test suite (test.sh), and there were no errors.

looks good....ping me when travis comes back green
 merged via https://github.com/pydata/pandas/commit/8517c05a83899bac0f6f79535723108f7d79df6a

thanks!
