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)
Status Job ID Name Coverage
  Test
passed checks-py2 #208783

00:03:09

passed checks-py3 #208784

00:03:23

passed doc #208789

00:00:33

manual pytype #208782
allowed to fail manual
passed tests-py2-cext #208785

00:07:28

passed tests-py2-pure #208786

00:07:08

passed tests-py3-cext #208787

00:07:26

passed tests-py3-pure #208788

00:06:40