clean: check that there are no conflicts after As noted by Pulkit, there should never be any conflicts after doing a clean update, so `hg.clean()` should never return `True`. Let's check that assertion instead to clarify the code. The callers will now get a `None` instead of a `False` returned, but that should be fine (both result in a 0 exit status). Differential Revision: https://phab.mercurial-scm.org/D7984