Remove extra cli flag, and update test case

This commit uses capsys to test the output of the diff, which is now
hidden behind the autofix flag if it's disabled

pre_commit_hooks/pretty_format_json.py

@@ -56,12 +56,12 @@ def parse_topkeys(s):  # type: (str) -> List[str]
     return s.split(',')
 
 
-def get_diff(source, target):  # type: (List[str], List[str]) -> str
-    source_lines = ''.join(source).split('\n')
-    target_lines = ''.join(target).split('\n')
-    d = difflib.Differ()
-    diff = d.compare(source_lines, target_lines)
-    return '\n'.join(diff)
+def get_diff(source, target):  # type: (str, str) -> str
+    source_lines = source.splitlines(True)
+    target_lines = target.splitlines(True)
+    diff = ''.join(difflib.ndiff(source_lines, target_lines))
+    print(diff)
+    return diff
 
 
 def main(argv=None):  # type: (Optional[Sequence[str]]) -> int
@@ -105,14 +105,6 @@ def main(argv=None):  # type: (Optional[Sequence[str]]) -> int
         default=[],
         help='Ordered list of keys to keep at the top of JSON hashes',
     )
-    parser.add_argument(
-        '--show-expected',
-        action='store_true',
-        dest='show_expected',
-        default=False,
-        help='Show a diff between the input file and expected (pretty) output',
-    )
-
     parser.add_argument('filenames', nargs='*', help='Filenames to fix')
     args = parser.parse_args(argv)
 
@@ -131,11 +123,10 @@ def main(argv=None):  # type: (Optional[Sequence[str]]) -> int
             if contents != pretty_contents:
                 print('File {} is not pretty-formatted'.format(json_file))
 
-                if args.show_expected:
-                    print(get_diff(contents, list(pretty_contents)))
-
                 if args.autofix:
                     _autofix(json_file, pretty_contents)
+                else:
+                    print(get_diff(''.join(contents), pretty_contents))
 
                 status = 1
         except ValueError: