rust-nodemap: core implementation for shortest In this implementation, we just make `lookup()` return also the number of steps that have been needed to come to a conclusion from the nodetree data, and `validate_candidate()` takes care of the special cases related to `NULL_NODE`. This way of doing minimizes code duplication, but it means that the comparatively slower finding of first non zero nybble will run for all calls to `find()` where it is not needed. Still running on the file generated for the mozilla-central repository, it seems indeed that we now get more ofter 320 ns than 310. The odds that this could have a significant impact on real life Mercurial performance are still looking low. Let's wait for actual benchmark runs to see if an optimization is needed here. Differential Revision: https://phab.mercurial-scm.org/D7819