pick: make sure the topic of cset being picked doesn't change

Now we feed the "current active topic" to context being committed only
when it doesn't already have a topic.

Changes in test file reflect a fix in pick command where the topic of
picked cset got changed to the repo's current active topic.
8 jobs for topic/default/pick-series in 8 minutes and 20 seconds (queued for 1 minute and 15 seconds)