templatekw: avoid slow creation of changectx objects in showgraphnode()

This mitigates the minor perf regression introduced by the previous patch.

  % hg log -G -R mozilla-central -l10000 --time > /dev/null
  (original) real 2.200 secs
  (previous) real 2.590 secs
  (this)     real 2.280 secs

mercurial/templatekw.py

@@ -7,7 +7,7 b''
 
 from __future__ import absolute_import
 
-from .node import hex
+from .node import hex, nullid
 from . import (
     error,
     hbisect,
@@ -343,7 +343,9 b' def showfiles(**args):'
 def showgraphnode(repo, ctx, **args):
     """:graphnode: String. The character representing the changeset node in
     an ASCII revision graph"""
-    wpnodes = [pctx.node() for pctx in repo[None].parents()]
+    wpnodes = repo.dirstate.parents()
+    if wpnodes[1] == nullid:
+        wpnodes = wpnodes[:1]
     if ctx.node() in wpnodes:
         return '@'
     elif ctx.obsolete():