Peru: They have quarreled over the 1880's pillaging of Peru's national library by Chilean troops. They have squabbled over who has the naming rights to pisco, the fiery grape brandy. Now, Peru and Chile are arguing over another hot-button issue: the origins of the potato. Peruvian agronomists, historians and diplomats are chafing at an assertion by Marigen Hornkohl, Chile's agriculture minister, who said on Monday, "Few people know that 99% of the world's potatoes have some type of genetic link to potatoes from Chile.' Peru, where the potato is a