historycache sets _merge_sorted_revisions_cache on a branch to a single-use iterator

I don't really know what the intent of the code is, but this code:

        if br._merge_sorted_revisions_cache is None:
            stored_data = cache.iter()
            if stored_data is not None:
                mutter("loaded history cache from %s" % cache.file_name())
                br._merge_sorted_revisions_cache = stored_data

appears to set br._merge_sorted_revisions_cache to an object of type 'listiter' -- which cannot possible go well, as by the time it gets accessed again, the iterator has been exhausted.