Change '== None' to 'is None'

contrib/devtools/clang-format-diff.py

@@ -109,7 +109,7 @@ def main():
     match = re.search('^\+\+\+\ (.*?/){%s}(\S*)' % args.p, line)
     if match:
       filename = match.group(2)
-    if filename == None:
+    if filename is None:
       continue
 
     if args.regex is not None: