Added support for 'arc diff --update'

Hey, I added just a small enhancement to allow for the --update flag to arc diff, thought you might find this useful as well. So am just trying this cross-fork pull-request. (github tells me : "We can’t automatically merge these branches. Don't worry, you can still submit the pull request." ...so feel free to just manually apply the changes to 
git-flow-feature and close this request, if you want to).