What version are you running?
3.0.17
What's the URL of the page containing the problem?

What steps will reproduce the problem?
Submit a code review where a line change includes a Windows "smart quote", such as 
FOO = 1, //Testing Window<92>s smart quote
What is the expected output? What do you see instead?
Instead of seeing the diff, I see this error:
 There was an error displaying this diff.
'utf8' codec can't decode byte 0x92 in position 530: invalid start byte
 This may be a a temporary outage or an issue with the format of your diff.
 Please try again, and if you still have trouble,
 contact support.

What operating system are you using? What browser?
 Ubuntu 18.04


Please provide any additional information below.
Stack trace:
Traceback (most recent call last):

  File "/usr/local/lib/python2.7/dist-packages/reviewboard/diffviewer/views.py", line 345, in get

    response = renderer.render_to_response(request)

  File "/usr/local/lib/python2.7/dist-packages/reviewboard/diffviewer/renderers.py", line 58, in render_to_response

    return HttpResponse(self.render_to_string(request))

  File "/usr/local/lib/python2.7/dist-packages/reviewboard/diffviewer/renderers.py", line 76, in render_to_string

    large_data=True)

  File "/usr/local/lib/python2.7/dist-packages/djblets/cache/backend.py", line 299, in cache_memoize

    compress_large_data))

  File "/usr/local/lib/python2.7/dist-packages/djblets/cache/backend.py", line 253, in cache_memoize_iter

    items = items_or_callable()

  File "/usr/local/lib/python2.7/dist-packages/djblets/cache/backend.py", line 296, in <lambda>

    lambda: [lookup_callable()],

  File "/usr/local/lib/python2.7/dist-packages/reviewboard/diffviewer/renderers.py", line 75, in <lambda>

    lambda: self.render_to_string_uncached(request),

  File "/usr/local/lib/python2.7/dist-packages/reviewboard/diffviewer/renderers.py", line 89, in render_to_string_uncached

    request=request)

  File "/usr/local/lib/python2.7/dist-packages/reviewboard/diffviewer/diffutils.py", line 786, in populate_diff_chunks

    chunks = list(generator.get_chunks())

  File "/usr/local/lib/python2.7/dist-packages/reviewboard/diffviewer/chunk_generator.py", line 786, in get_chunks

    for chunk in super(DiffChunkGenerator, self).get_chunks(cache_key):

  File "/usr/local/lib/python2.7/dist-packages/reviewboard/diffviewer/chunk_generator.py", line 107, in get_chunks

    large_data=True)

  File "/usr/local/lib/python2.7/dist-packages/djblets/cache/backend.py", line 299, in cache_memoize

    compress_large_data))

  File "/usr/local/lib/python2.7/dist-packages/djblets/cache/backend.py", line 253, in cache_memoize_iter

    items = items_or_callable()

  File "/usr/local/lib/python2.7/dist-packages/djblets/cache/backend.py", line 296, in <lambda>

    lambda: [lookup_callable()],

  File "/usr/local/lib/python2.7/dist-packages/reviewboard/diffviewer/chunk_generator.py", line 106, in <lambda>

    lambda: list(self.get_chunks_uncached()),

  File "/usr/local/lib/python2.7/dist-packages/reviewboard/diffviewer/chunk_generator.py", line 793, in get_chunks_uncached

    new = get_patched_file(old, self.filediff, self.request)

  File "/usr/local/lib/python2.7/dist-packages/reviewboard/diffviewer/diffutils.py", line 309, in get_patched_file

    request=request)

  File "/usr/local/lib/python2.7/dist-packages/reviewboard/diffviewer/diffutils.py", line 185, in patch

    if empty_diff(diff):

  File "/usr/local/lib/python2.7/dist-packages/reviewboard/diffviewer/diffutils.py", line 126, in empty_diff

    arr = diff.decode('utf8').split('\n',10)

  File "/usr/lib/python2.7/encodings/utf_8.py", line 16, in decode

    return codecs.utf_8_decode(input, errors, True)

UnicodeDecodeError: 'utf8' codec can't decode byte 0x92 in position 530: invalid start byte