The best side dish you haven't had yet. At Barbs-B-Q in Lockhart, Texas, it's listed as 'green spaghett.'Credit: Robbie CaponettoIf you read the title of this piece and thought of the faintly green, often spinach-dyed linguine you can find in some pasta aisles, then keep reading. What we’re talking about is the deeply green, almost neon-bright noodles you’ll see served at Mexican potlucks, and more rarely as a barbecue side in Texas.The dish has…