shortest: move revnum-disambiguation out of revlog I want to be able to change how we disambiguate and I think having revlog.shortest() worry only about finding a prefix that's unambiguous among nodeids makes more sense. This slows down `hg log -T '{shortest(node,1)}\n'` from 4.0s to 4.1s. Differential Revision: https://phab.mercurial-scm.org/D3502