Removing empty directories doesn't work

[talex5]

Because Git doesn't store empty directories, mkdir in a transaction's rw directory doesn't change the tree. However, because we do want the new directory to appear in ls listings we add it to extra_dirs.

However, when we remove a directory we don't delete it from its parent's extra_dirs and it still appears in the ls output.

[samoht]

I've just hit that and it is a bit surprising :-)

[samoht]

@talex5 any chance you could have a look at it? It makes some stuff in the bridge a bit tricky..