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
#require docutils gettext
$ $TESTDIR/check-gendoc sv
checking for parse errors