Return 1 if --diff changed the code.

This is how GNU diff acts.

CHANGELOG

@@ -5,6 +5,7 @@
 ## [0.16.4] Unreleased
 ### Changed
 - Adjust blank lines on formatting boundaries when using the `--lines` option.
+- Return 1 if a diff changed the code. This is in line with how GNU diff acts.
 
 ## [0.16.3] 2017-07-13
 ### Changed

yapf/__init__.py

@@ -177,7 +177,7 @@ def main(argv):
   if not files:
     raise errors.YapfError('Input filenames did not match any python files')
 
-  FormatFiles(
+  changed = FormatFiles(
       files,
       lines,
       style_config=args.style,
@@ -186,7 +186,7 @@ def main(argv):
       print_diff=args.diff,
       verify=args.verify,
       parallel=args.parallel)
-  return 0
+  return 1 if changed and args.diff else 0
 
 
 def FormatFiles(filenames,

yapftests/yapf_test.py

@@ -492,7 +492,10 @@ def f():
         suffix='.py', dirname=self.test_tmpdir) as (out, _):
       with utils.TempFileContents(
           self.test_tmpdir, unformatted_code, suffix='.py') as filepath:
-        subprocess.check_call(YAPF_BINARY + ['--diff', filepath], stdout=out)
+        try:
+          subprocess.check_call(YAPF_BINARY + ['--diff', filepath], stdout=out)
+        except subprocess.CalledProcessError as e:
+          self.assertEqual(e.returncode, 1)  # Indicates the text changed.
 
   def testReformattingSpecificLines(self):
     unformatted_code = textwrap.dedent("""\