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.
Upstream-commit: 727e7fcccadf1d3e286f5a3c8d1aa388f6b4dab8
Component: engine