That way we can join forks-of-forks into the same network even if
their original source was deleted.

Consider the following:

  `awesome-oss/badger` is forked to `coolstuff/badger`, which is
  forked to `user-a/badger` which in turn is forked to
  `user-b/badger`.

  When `awesome-oss/badger` is deleted, we will now create a fork
  network with `coolstuff/badger` as the root. The `user-a/badger`
  and `user-b/badger` projects will be added to the network.