Change the comparison to better handle files that are copied during
container creation but not actually changed:
* Inode - this will change during a copy
* ctime - this will change during a copy (as we can't set it back)
* blocksize - this will change for sparse files during copy
* size for directories - this can change anytime but doesn't
necessarily reflect an actual contents change
* Compare mtimes at microsecond precision (as this is what utimes has)
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.
