topic: add `--prune` to the topic command

This is a more discoverable option than the `--topic` option on the `prune`
command (though that gives more options) due to its proximity to `--clear`.

I'm sure there's a way to build the `**opts` for the command less manually, but
I don't know how.
8 jobs for topic/default/topic_prune in 3 minutes and 53 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Test
passed checks-py2 #93393

00:01:49

passed checks-py3 #93394

00:01:54

passed doc #93399

00:00:18

manual pytype #93392
allowed to fail manual
failed tests-py2-cext #93395

00:03:53

failed tests-py2-pure #93396

00:03:28

failed tests-py3-cext #93397

00:02:21

failed tests-py3-pure #93398

00:03:18

 
Name Stage Failure
failed
tests-py3-pure Test
Skipped test-check-pyflakes.t: blacklisted
Skipped test-check-setup-manifest.t: blacklisted
Failed test-split.t: output changed
Failed test-topic.t: output changed
# Ran 193 tests, 6 skipped, 2 failed.
python hash seed: 4039157627
Running after_script
Uploading artifacts for failed job
ERROR: Job failed: exit code 1
failed
tests-py2-pure Test
Skipped test-check-pyflakes.t: blacklisted
Skipped test-check-setup-manifest.t: blacklisted
Failed test-split.t: output changed
Failed test-topic.t: output changed
# Ran 193 tests, 6 skipped, 2 failed.
python hash seed: 4042377296
Running after_script
Uploading artifacts for failed job
ERROR: Job failed: exit code 1
failed
tests-py2-cext Test
Skipped test-check-pyflakes.t: blacklisted
Skipped test-check-setup-manifest.t: blacklisted
Failed test-split.t: output changed
Failed test-topic.t: output changed
# Ran 193 tests, 6 skipped, 2 failed.
python hash seed: 1298224891
Running after_script
Uploading artifacts for failed job
ERROR: Job failed: exit code 1
failed
tests-py3-cext Test
Skipped test-check-pyflakes.t: blacklisted
Skipped test-check-setup-manifest.t: blacklisted
Failed test-split.t: output changed
Failed test-topic.t: output changed
# Ran 193 tests, 6 skipped, 2 failed.
python hash seed: 3628924062
Running after_script
Uploading artifacts for failed job
ERROR: Job failed: exit code 1