mercurial/templates/json/graph.tmpl

relnotes: enable extension when running releasenotes command

Last night, when I was working on different machine, the test worked, now on a
different machine it does not. Looks like we should make sure releasenotes
extension is loaded.

Differential Revision: https://phab.mercurial-scm.org/D4425

\{
  "node": {node|json},
  "changeset_count": {changesets|json},
  "changesets": [{join(nodes%graphentry, ", ")}]
}