I assume you don't want to lose your change history. That is the hard part.

as Geoff said

using the scm commandline command you could write a script that
(you will lost the original date/time I think. )

creates the new repo (git init xyz)
lists the rtc changesets  (scm list changesets use sort for reverse order)
loop thru the changesets 
    extract one changeset at a time  (scm changeset extract)
       have to figure out a git add/rm approach for all the changes in a commit
    use the git commit command to replicate that into git
    use git tag to map to rtc baselines.
end loop
git push

How were the stream migration handled ? Was there a individual stream to branch mapping ?

thanks,
Rohit

The original post is almost 2 yrs ago now. I don't think there has been a solution created