Bug#980900: New watch file for Graphviz package v2

[Matthew Fernandez]

It didn’t occur to me to mention before, but the Graphviz download web 
page is also driven programmatically from some JSON files: 
https://gitlab.com/graphviz/graphviz.gitlab.io/-/tree/main/data/releases?ref_type=heads. 
Another option may be for the watch file to refer to JSON files in this 
Gitlab repo. Though perhaps it is required for a watch file to refer to 
a source tarball, not something indirect.

The Graphviz repo at https://gitlab.com/graphviz/graphviz also has Git 
tags for every release.