revlog: use node tree (native code) for shortest() calculation I want to rewrite revlog.shortest() to disambiguate only among hex nodeids and then disambiguate the result with revnums at a higher level (in scmutil). However, that would slow down `hg log -T '{shortest(node,1)}\n'` from 5.0s to 6.8s, which I wasn't sure would be acceptable. So this patch makes revlog.shortest() use the node tree for finding the length of the shortest prefix that's unambiguous among nodeids. Once that has been found, it makes it longer until it is also not ambiguous with a revnum. This speeds up `hg log -T '{shortest(node,1)}\n'` from 5.0s to 4.0s. Differential Revision: https://phab.mercurial-scm.org/D3499