node: add 'nullhex', hex-encoded nullid

We're going to need this for upcoming changes, because the merge state stores
nodes as hex strings.

mercurial/node.py

@@ -9,17 +9,18 b' from __future__ import absolute_import'
 
 import binascii
 
+# This ugly style has a noticeable effect in manifest parsing
+hex = binascii.hexlify
+bin = binascii.unhexlify
+
 nullrev = -1
 nullid = "\0" * 20
+nullhex = hex(nullid)
 
 # pseudo identifiers for working directory
 # (they are experimental, so don't add too many dependencies on them)
 wdirrev = 0x7fffffff
 wdirid = "\xff" * 20
 
-# This ugly style has a noticeable effect in manifest parsing
-hex = binascii.hexlify
-bin = binascii.unhexlify
-
 def short(node):
     return hex(node[:6])