Efforts to discredit government informants are nothing new in Philadelphia’s decades-long history of blockbuster Mafia trials — a catalog built on testimony from a rogues’ gallery of mob turncoats and wiseguys-turned-witnesses with rap sheets rife with material for any cross-examining defense lawyer. Investigators often say that they don’t have the luxury of choosing saints as their witnesses, and that only through informants with questionable pasts are they able to net larger targets.