hg: use mercurial.node.nullid directly - needed for 5.9

mercurial.commands.nullid was removed in d55b71393907.

kallithea/lib/vcs/backends/hg/repository.py

@@ -212,7 +212,7 b' class MercurialRepository(BaseRepository'
         local = False
 
         try:
-            mercurial.tags.tag(self._repo, safe_bytes(name), mercurial.commands.nullid, safe_bytes(message), local, safe_bytes(user), date)
+            mercurial.tags.tag(self._repo, safe_bytes(name), mercurial.node.nullid, safe_bytes(message), local, safe_bytes(user), date)
             self.tags = self._get_tags()
         except mercurial.error.Abort as e:
             raise RepositoryError(e.args[0])