NUEVO LAREDO, Mexico (Reuters) - Thousands of expatriate Mexicans streamed south of the U.S. border on Sunday to vote in their homeland in a tight race with high stakes for crime-weary border residents. A large stream of U.S.-based Mexicans trekked on foot and piled into cars to vote in a string of gritty border towns from Tijuana in the west to Nuevo Laredo below Texas. Electoral authorities were taken by surprise at the number of expatriates who showed up. "It's a very close-fought race ... and if we don't vote, we can't hope to decide the outcome," said Luis Tovar,...