Comment 6 for bug 715000

It looks like get_parent_map and _get_parent_map_with_sources could have a similar bug. Perhaps adding a function to get the full expansion, then checking which callers should be updated to use this seems better. Rewriting every function to recurse through the public interface seems to have more danger of causing performance problems.

Doing so seems to fix my test for it, assuming it's representative.

If we're comfortable with this fix, we should also grep for _fallback_vfs and update them, and perhaps even rename that to something like _immediate_fallbacks that won't encourage people to make the same mistake.