Very slow directory traversal

[Alex Bligh]

> All the large directories are indexed, but some smaller or empty ones
> seem not to be.

I think that's correct; it doesn't build the index tree until the directory
reaches (from memory) a couple of blocks.

I vaguely recall that one can still use readdir / telldir and end up
with an O(n^2) result, but I forget how. You've reached the limit of
my knowledge here.

Alex