CONTRIBUTING

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

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