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
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