mercurial/templates/json/graph.tmpl

unionrepo: use a lower level overide in unionrepo too

The unionrepo class also have a strange `baserevision` hack, let's try to get
ride of it too.

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