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
rebase: use fm.formatlist() and fm.formatdict() to support user template
Thanks to Yuya for suggesting this in D1173.
Differential Revision: https://phab.mercurial-scm.org/D1293