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

revlog: guarantee that p1 != null if a non-null parent exists This change does not affect the hashing (which already did this transformation), but can change the log output in the rare case where this behavior was observed in repositories. The change can simplify iteration code where regular changesets and merges are distinct branches. Differential Revision: https://phab.mercurial-scm.org/D10150