histedit: add support to output nodechanges using formatter
The JSON output of nodechanges will help in automation and to improve editor
integrations such as for Nuclide.
Differential Revision: https://phab.mercurial-scm.org/D1348
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}