Speed up roundtrip syntax tests

Running the unified diff is really slow, so first check if the strings
are identical. Speeds up the roundtrip stdlib test by 8x on my machine.

Author: benlangmuir

utils/round-trip-syntax-test

@@ -76,9 +76,12 @@ class RoundTripTask(object):
 
         contents = ''.join(map(lambda l: l.decode('utf-8', errors='replace'),
                                open(self.input_filename).readlines()))
-        lines = difflib.unified_diff(contents,
-                                     self.stdout.decode('utf-8',
-                                                        errors='replace'),
+        stdout_contents = self.stdout.decode('utf-8', errors='replace')
+
+        if contents == stdout_contents:
+          return None
+
+        lines = difflib.unified_diff(contents, stdout_contents,
                                      fromfile=self.input_filename,
                                      tofile='-')
         diff = '\n'.join(line for line in lines)