obslog: add preliminary support for showing predecessors

This functionality is not yet used right now, but will be in the next patch.
7 jobs for topic/default/obslog-predecessors-1 in 3 minutes and 41 seconds (queued for 1 second)
Status Name Job ID Coverage
  Test
passed doc #69328

00:00:31

passed flake8 #69322

00:00:17

manual pytype #69323
allowed to fail manual
passed tests-py2-cext #69324

00:03:28

passed tests-py2-pure #69325

00:03:41

passed tests-py3-cext #69326

00:03:36

passed tests-py3-pure #69327

00:03:18