shortest: don't include nullid in disambigution revset As noted by Yuya in the review of D4118, the fact that the nodetree always contains the nullid is not what we want for the disambiguation case. It's not what the pure version of the code does. Note that the updated test also passes before this patch, but it wouldn't have passed in native-code mode once we start using the nodetree for disambiguating nodeid prefixes. Differential Revision: https://phab.mercurial-scm.org/D4163