Rather than scan the files in the old directory twice to detect the
deletions we now scan both directories twice and then do all the
diffing on the in-memory structure.
This is more efficient, but it also lets us diff more complex things
later that are not exact on-disk trees.
To do diffing we just compare file metadata, so this relies
on things like size and mtime/ctime to catch any changes.
Its *possible* to trick this by updating a file without
changing the size and setting back the mtime/ctime, but
that seems pretty unlikely to happen in reality, and lets
us avoid comparing the actual file data.
