Add tag write access to merge script.

tools/push-to-trunk/git_recipes.py

 #!/usr/bin/env python
 # Copyright 2014 the V8 project authors. All rights reserved.
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 #       notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 #       copyright notice, this list of conditions and the following
@@ skipping 62 matching lines @@
     footers[m.group(1)] = m.group(2).strip()
   return footers
 
 
 class GitFailedException(Exception):
   pass
 
 
 def Strip(f):
   def new_f(*args, **kwargs):
-    return f(*args, **kwargs).strip()
+    result = f(*args, **kwargs)
+    if result is None:
+      return result
+    else:
+      return result.strip()
   return new_f
 
 
 def MakeArgs(l):
   """['-a', '', 'abc', ''] -> '-a abc'"""
   return " ".join(filter(None, l))
 
 
 def Quoted(s):
   return "\"%s\"" % s
@@ skipping 211 matching lines @@
   def GitSVNFindSVNRev(self, git_hash, branch="", **kwargs):
     return self.Git(MakeArgs(["svn find-rev", git_hash, branch]), **kwargs)
 
   def GitSVNDCommit(self, **kwargs):
     return self.Git("svn dcommit 2>&1", retry_on=lambda x: x is None, **kwargs)
 
   def GitSVNTag(self, version, **kwargs):
     self.Git(("svn tag %s -m \"Tagging version %s\"" % (version, version)),
              retry_on=lambda x: x is None,
              **kwargs)