Directories are hashed too

Since file bar didn't change, its hash didn't change

So it needn't be saved again

Nothing in doc changed, so nothing in there needs to be saved again

And doc itself is identical and needn't be saved again

Only the new versions of foo and the root tree need to be saved

Similarly, if I move or rename a file its contents need not be re-recorded

Only its containing directory has changed

Directories are tamper-proof