Though the program had its oddities and performances could be tentative, Christian Gerhaher left no doubt Thursday at his Philadelphia debut recital that he possesses a Stradivarius among voices and is one of the most cultivated singers in the new generation of German art-song interpreters. You could have guessed that by the Andras Schiff stamp of approval: The in-demand pianist only occasionally accompanies singers, but there he was, playing a secondary role to this Bavaria-born baritone, so little known in the United States.