Right now, it's not at all obvious that we *expect* a star topology whenever
you're using a foreign replica, and it's all too easy to accidentally pull or
push to a new foreign replica. We need to make it absolutely clear that SD will
become very confused if more than one person is talking to a legacy system and
sharing these changes with others.
* add --force whenever a user tries to push or pull to a new foreign replica
she hasn't pushed or pulled from before, with a scary warning message
for SD