prop140: Handle signatures correctly

For diffs to work properly, we need to check the input document and the output document in their entirety, including their signatures. Otherwise, the diffs won't apply correctly when they change the signatures!

But for that to work, we need to do what we can to minimize the odds that anybody has a consensus with different signatures, or with signatures organized differently.

As an alternative, we could change the diff format so that it always replaces all the old signatures with the new ones.