Allow existing repo to become a fork (BB-509)

It would be nice if an existing repo could later become a fork of another project after the fact. For instance, tksoh just moved his TortoiseHg repos to bitbucket and now I would like to make my thg-hg-crew-tip repo a fork of this (albeit at a given rev).

I know I could just kill and recreate the repo, but that would throw followers off, wouldn't it?

http://www.bitbucket.org/tksoh/tortoisehg-crew/overview/

http://www.bitbucket.org/parren/thg-hg-crew-tip/overview/