Activating Gundo makes __Gundo_Preview__ the alternate file
After activating and deactivating Gundo, the alternate file becomes Gundo_Preview. That means that
:e# opens the most recent Gundo preview diff instead of the last file I was working with. That's kind of annoying. On the other hand, I can't figure out how the heck you can tell Vim not to do it. It's already nobuflisted. Possibly judicious use of :keepalt, though that seems tricky. But I'm posting this issue in case someone has a good idea.