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

--- a/relnotes/next	Wed Mar 10 18:09:21 2021 +0100
+++ b/relnotes/next	Wed Jan 13 16:14:58 2021 +0100
@@ -43,3 +43,7 @@
    now get a revision number as argument instead of a node.
 
  * revlog.addrevision returns the revision number instead of the node.
+
+ * `nodes.nullid` and related constants are being phased out as part of
+   the deprecation of SHA1. Repository instances and related classes
+   provide access via `nodeconstants` and in some cases `nullid` attributes.