discovery: list the new heads like core does when failing a multi-head push

The message was, and still is slightly different from core (which mentions
'remote' heads).  But it's probably useful to have slightly different messages
so that we can easily determine the source.
8 jobs for topic/default/topic-list-push-heads in 4 minutes and 4 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed checks-py2 #111220

00:02:02

passed checks-py3 #111221

00:00:53

passed doc #111226

00:00:34

manual pytype #111219
allowed to fail manual
passed tests-py2-cext #111222

00:04:04

passed tests-py2-pure #111223

00:03:27

passed tests-py3-cext #111224

00:03:47

passed tests-py3-pure #111225

00:02:48