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.