phabricator: add debug logging to show previous node values in `phabsend` This isn't real useful here, but was very useful showing how `phabsend --fold` handles commits created by `hg fold` and `hg split`. It introduces a new debug function and flag instead of using `ui.debug()`, because `--debug` prints out all of the API chatter. Differential Revision: https://phab.mercurial-scm.org/D8389