That is where the last refactorings were leading. This code is
simpler, but it's far more flexible. Importantly, it only makes
a single copy instead of making a full disk copy for every
intermediate step.
Next we're going to make the "copy" part modular for processes
that don't need to copy the whole file.
Change-Id: Ief32ac665d804b9b20c44f443a9c87452ceb367a
ae55db95a7