refactor: prefer checks against nullrev over nullid A common pattern is using a changeset context and obtaining the node to compare against nullid. Change this to obtain the nullrev instead. In the future, the nullid becomes a property of the repository and is no longer a global constant, so using nullrev is much easier to reason about. Python function call overhead makes the difference moot, but future changes will result in more dictionary lookups otherwise, so prefer the simpler pattern. Differential Revision: https://phab.mercurial-scm.org/D10290