We are trying to move all HHVM development from Facebook's internal phabricator instance to our public http://reviews.facebook.net/ . HHVM lives in a subdirectory inside of a larger git directory (fbcode, if you remember). We would like to put a `.arcconfig` in the `hphp` subdirectory and then have diffs applied to that directory go to the open source phabricator. If a diff spans any files outside, then I think it is totally reasonable to throw an exception and not let them upload that diff.