node: introduce nodeconstants class In preparing for moving from SHA1 hashes to a modern hash function, place nullid and other constant magic vules in a class. Provide the active set of constants in the repository and push it down. Provide nullid directly in strategic places like the repository as it is accessed very often. This changeset introduces the API change, but not the mechanical replacement of the node.py attributes itself. Differential Revision: https://phab.mercurial-scm.org/D9750