This parallels the implementation of windowsDiff.Apply, including
bouncing very briefly though archive.WriteDiff and then straight back
out into Windows-specific code.
It's mostly pulling existing mechanisms from non-Windows Compare or
Windows Apply, and highlights that there's probably a lot of scope for
refactoring on top of this.
Now the export-related integration tests pass CI on Windows.
Signed-off-by: Paul "TBBle" Hampson <Paul.Hampson@Pobox.com>
