Store SHA of each article + descendants and skip API re-renders for entire subtrees

This is one step beyond skip re-render from API if article was unchanged as it removes the requirement of actually uploading thousands of lines of content.

It requires negotiating with the server instead.

This would be particularly powerful if we included the descendants on the SHA of each parent, much like Git. This way we could skip enter unmodified subtrees, likely like Git.

Yes, we are somewhat re-implementing parts of Git with this. But at least it is simple, and works at a sub-blob level given our grater specialization to our specific use case.