repo: look up nullrev context by revnum, not symbolic name

I think lookup of the 'null' symbol should be done via
scmutil.revsymbol() and repo['null'] is only supported for historical
reasons. However, repo[nullrev] is fine, so we can switch to that
instead of switching to scmutil.revsymbol('null').

Differential Revision: https://phab.mercurial-scm.org/D4780

mercurial/archival.py

@@ -16,6 +16,9 b' import zipfile'
 import zlib
 
 from .i18n import _
+from .node import (
+    nullrev,
+)
 
 from . import (
     error,
@@ -76,7 +79,7 b' def _rootctx(repo):'
     # repo[0] may be hidden
     for rev in repo:
         return repo[rev]
-    return repo['null']
+    return repo[nullrev]
 
 # {tags} on ctx includes local tags and 'tip', with no current way to limit
 # that to global tags.  Therefore, use {latesttag} as a substitute when

mercurial/scmutil.py

@@ -21,6 +21,7 b' from .node import ('
     bin,
     hex,
     nullid,
+    nullrev,
     short,
     wdirid,
     wdirrev,
@@ -730,7 +731,7 b' def meaningfulparents(repo, ctx):'
     if len(parents) > 1:
         return parents
     if repo.ui.debugflag:
-        return [parents[0], repo['null']]
+        return [parents[0], repo[nullrev]]
     if parents[0].rev() >= intrev(ctx) - 1:
         return []
     return parents