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.
Upstream-commit: 1c5dc26a7c0a0abb7bc59174768ec309f6c5fd4f
Component: engine
