Fix option --diff-only not working on stdin input

1. Fix diff reporting not working on stdin input
2. Alias --diff option to --diff-only

reorder_python_imports.py

@@ -482,7 +482,10 @@ def _fix_file(filename: str, args: argparse.Namespace) -> int:
         unclassifiable_application_modules=args.unclassifiable,
     )
     if filename == '-':
-        print(new_contents, end='')
+        if args.diff_only:
+            _report_diff(contents, new_contents, '')
+        else:
+            print(new_contents, end='')
     elif contents != new_contents:
         if args.diff_only:
             _report_diff(contents, new_contents, filename)
@@ -510,7 +513,7 @@ def _report_diff(contents: str, new_contents: str, filename: str) -> None:
             fromfile=filename, tofile=filename,
         ),
     )
-    if not diff.endswith('\n'):
+    if diff and not diff.endswith('\n'):
         diff += '\n\\ No newline at end of file\n'
 
     print(diff, end='')

tests/reorder_python_imports_test.py

@@ -744,9 +744,10 @@ def test_integration_main_stdout(tmpdir, capsys):
     )
 
 
-def _apply_patch(patch):
-    patch_proc = subprocess.Popen(('patch',), stdin=subprocess.PIPE)
-    patch_proc.communicate(patch.encode('UTF-8'))
+def _apply_patch(patch, origfile=None):
+    args = ('patch', origfile) if origfile else ('patch',)
+    patch_proc = subprocess.Popen(args, stdin=subprocess.PIPE)
+    patch_proc.communicate(patch.encode())
     assert patch_proc.returncode == 0
 
 
@@ -1135,6 +1136,27 @@ def test_main_stdin_no_fix(capsys):
     assert out == 'import os\nimport sys\n'
 
 
+def test_main_stdin_diff_only(tmpdir, capsys):
+    tf = tmpdir.join('t.py')
+    input_b = b'import sys\nimport os\n'
+    tf.write(input_b)
+    stdin = io.TextIOWrapper(io.BytesIO(input_b), 'UTF-8')
+    with mock.patch.object(sys, 'stdin', stdin):
+        assert main(('-', '--diff-only')) == 1
+    out, _ = capsys.readouterr()
+    _apply_patch(out, origfile=tf)
+    assert tf.read() == 'import os\nimport sys\n'
+
+
+def test_main_stdin_diff_only_no_changes(capsys):
+    input_b = b'import os\nimport sys\n'
+    stdin = io.TextIOWrapper(io.BytesIO(input_b), 'UTF-8')
+    with mock.patch.object(sys, 'stdin', stdin):
+        assert main(('-', '--diff-only')) == 0
+    out, err = capsys.readouterr()
+    assert out == ''
+
+
 def test_main_exit_code_multiple_files(tmpdir):
     f1 = tmpdir.join('t1.py')
     f1.write('import os,sys\n')