Efficient Transfer Hydrogenation of Ketones Catalyzed by the Bis(isocyanide)−Ruthenium(II) Complexes trans,cis,cis-[RuX2(CNR)2(dppf)] (X = Cl, Br; dppf = 1,1‘-Bis(diphenylphosphino)ferrocene): Isolation of Active Mono- and Dihydride Intermediates†