Fix the reported HP difference for -sethp.
Fix the reported HP difference for -sethp.

-sethp would report the percentage of remaining HP as the difference. This would also always result in a positive delta, even if HP was lost.

asgdf/Pokemon-Showdown-Client,submindraikou/Pokemon-Showdown-Client,sirDonovan/Pokemon-Showdown-Client,Zarel/Pokemon-Showdown-Client,panpawn/Gold-Client,sirDonovan/Pokemon-Showdown-Client,QuiteQuiet/Pokemon-Showdown-Client,sirDonovan/Pokemon-Showdown-Client,Zarel/Pokemon-Showdown-Client,QuiteQuiet/Pokemon-Showdown-Client,Zarel/Pokemon-Showdown-Client,asgdf/Pokemon-Showdown-Client,panpawn/Gold-Client,submindraikou/Pokemon-Showdown-Client,submindraikou/Pokemon-Showdown-Client,QuiteQuiet/Pokemon-Showdown-Client,Zarel/Pokemon-Showdown-Client,asgdf/Pokemon-Showdown-Client,sirDonovan/Pokemon-Showdown-Client,asgdf/Pokemon-Showdown-Client,panpawn/Gold-Client,QuiteQuiet/Pokemon-Showdown-Client