Fixed crash on non-ascii diff

[thk123]

Sometimes the diff file would contain a unicode character. Since we were opening the file, this meant that unidiff was reading the file in the wrong format. By letting unidiff handle reading the file we slightly simplify the code and fix this problem.

[thk123]

Error that prompted this:

The linter output is below (empty output means no errors).
Traceback (most recent call last):
  File "scripts/filter_lint_by_diff.py", line 15, in <module>
    diff = unidiff.PatchSet(f)
  File "/usr/local/lib/python2.7/site-packages/unidiff/patch.py", line 278, in __init__
    self._parse(data, encoding=encoding)
  File "/usr/local/lib/python2.7/site-packages/unidiff/patch.py", line 320, in _parse
    current_file._parse_hunk(line, diff, encoding)
  File "/usr/local/lib/python2.7/site-packages/unidiff/patch.py", line 216, in _parse_hunk
    hunk.append(original_line)
  File "/usr/local/lib/python2.7/site-packages/unidiff/patch.py", line 132, in append
    s = str(line)
  File "/usr/local/lib/python2.7/site-packages/unidiff/patch.py", line 53, in <lambda>
    cls.__str__ = lambda x: x.__unicode__().encode(DEFAULT_ENCODING)
  File "/usr/local/lib/python2.7/site-packages/unidiff/patch.py", line 79, in __str__
    return "%s%s" % (self.line_type, self.value)
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc2 in position 79: ordinal not in range(128)