dot/gnus.el: Add dummy root for loose threads.

dot/gnus.el: Add dummy root for loose threads.

Gnus has a long-standing bug with `%B'-ish tree views and loose threads:
subsequent article trees are attached to the last article in the
previous tree, rather than to the original root, resulting in the tree
display marching off the right-hand side of the screen.

This doesn't seem to happen with the `dummy' strategy, which introduces
somewhat ugly false root nodes, but is otherwise fine.