In a large saucepan, heat the olive oil with the garlic over moderate heat. Add the stock and the saffron and bring to a boil. Add the mussels, cover the pan, and steam until all the mussels have opened, 3 to 3 minutes; discard any unopened mussels. With a slotted spoon, remove the mussels from the pan and keep them warm in a covered bowl. Add the tomatoes to the pan, breaking them up with your hands, and stir in the remaining ingredients, except for the chives. Simmer briskly until the sauce is thick but fairly liquid, 5 to 7 minutes. then return the mussels to the pan and simmer to warm them through, about 1 minute more. Spoon the sauce, shells and all, over cooked pasta. Garnish with chives.