Clean and rinse beans, then soak in about 6 cups of water for 6 to 8 hours. Discard soaking water, rinse beans and place in large pot with 4 cups fresh water, minced garlic, cumin and cayenne. Simmer until tender, about 1 hour. Heat 1/2 cup of water in a large skillet. Add onion and cook until soft, about 5 minutes. Stir in tomatoes and diced chilies. Cook uncovered on medium heat for 10 minutes, stirring occasionally. Add cooked beans, including some cooking liquid, a cup at a time to tomatoes mixture. Mash some of the beans as you add them. When all the beans have been added, stir to mix, then cook over low heat, stirring frequently utnil thickened.