[TxMt] How to rebase a fork-of-a-fork bundle on GitHub?

[Adam Vandenberg]

I have a fork of the Perforce bundle, except that I forked it from a
previous GitHub fork that did a SVN import.
When I try to follow the recent instructions for rebasing, I end up
with "no common commits" and no real idea of how to resolve that
situation.

Is there something I can try to get my fork rebased to the official
bundle (and get GitHub to note that), or am I better off just newly
forking the official mirror and replaying the changes?

-Adam Vandenberg
http://adamv.com/