WIP: Topic/default/topic prune

Open Matt Harbison requested to merge topic/default/topic_prune into branch/default

I need to figure out why the prune command seems to be missing only in test-split.t (seems similar to https://bz.mercurial-scm.org/show_bug.cgi?id=6039, which I still haven't been able to figure out). But before spending time on that, I want to make sure that this is the proper approach.

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