Some Hollywood feuds leave behind bruised egos, sabotaged careers, and even mustard stains. Jimmy Kimmel and Matt Damon, however, have spent the last twenty years doing the exact opposite. Their long-running rivalry has always been one elaborate joke, born from Kimmel's throwaway sign-off, originally invented to make a struggling late-night show end on a bigger laugh and keep audiences coming back..link { text-decoration: underline; font-weight:…