refactor: prefer checks against nullrev over nullid...
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
