[MERGE] Fetch into stacked branches works correctly

[Martin Pool]

I'd like to add a comment explaining why we shortcircuit back to the
generic RepoFetcher in this case.  Does this one seem reasonable, or
can you suggest something better

            # the pack fetcher assumes it can just copy pack binary data, but
            # that may not be safe if there are fallback repositories on
            # either side; instead we look at the revisions and deltas one by
            # one.


-- 
Martin <http://launchpad.net/~mbp/>