progress: extract function for closing topic
progress(None) had a completely different implementation from the
progress(<not None>) implementation. It very much feels like it should
be a separate method, so this patch makes it so. That also makes it
clear that only the topic parameter matters when closing a topic
(e.g. "total" does not matter).
Differential Revision: https://phab.mercurial-scm.org/D3821
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