GenGraphs does not produce deterministic output

The build tool jigsaw.GenGraphs generates .dot files for module graphs, which are further on consumed by dot (graphviz) to generate svg files, which are included in the documenation.

The svg files has been a source of non-determinism in the build; I have erroneously attributed this to the dot tool. It turned out that it is GenGraphs that are producing slightly varying .dot files instead.