Fix exclude in recursive mode

find_files did match files and subdirectories from excluded directory in recursive mode.

E.g. if running `autopep8 -i -r --exclude dir/excluded dir/` files from `dir/excluded/` and `dir/excluded/subdir/` were still taken in account.

BTW, there are 5 tests failing, but these are not related to my changes. I get the same failures without my commit.

    ======================================================================
    FAIL: test_diff (__main__.CommandLineTests)
    ----------------------------------------------------------------------
    Traceback (most recent call last):
      File "test\test_autopep8.py", line 4245, in test_diff
        self.assertEqual(fixed, '\n'.join(result.split('\n')[3:]))
    AssertionError: u"-'abc'  \n+'abc'\n" != u"-'abc'  \r\n+'abc'\r\n"
    - -'abc'
    + -'abc'
    ?         +
    - +'abc'
    + +'abc'
    ?       +


    ======================================================================
    FAIL: test_pep8_ignore (__main__.CommandLineTests)
    ----------------------------------------------------------------------
    Traceback (most recent call last):
      File "test\test_autopep8.py", line 4272, in test_pep8_ignore
        self.assertEqual(line, result)
    AssertionError: u"'abc'  \n" != u"'abc'  \r\n"
    - 'abc'
    + 'abc'
    ?        +


    ======================================================================
    FAIL: test_pep8_ignore_should_handle_trailing_comma_gracefully (__main__.CommandLineTests)
    ----------------------------------------------------------------------
    Traceback (most recent call last):
      File "test\test_autopep8.py", line 4278, in test_pep8_ignore_should_handle_trailing_comma_gracefully
        self.assertEqual(fixed, result)
    AssertionError: u"'abc'\n" != u"'abc'\r\n"
    - 'abc'
    + 'abc'
    ?      +


    ======================================================================
    FAIL: test_pep8_passes (__main__.CommandLineTests)
    ----------------------------------------------------------------------
    Traceback (most recent call last):
      File "test\test_autopep8.py", line 4267, in test_pep8_passes
        self.assertEqual(fixed, result)
    AssertionError: u"'abc'\n" != u"'abc'\r\n"
    - 'abc'
    + 'abc'
    ?      +


    ======================================================================
    FAIL: test_recursive (__main__.CommandLineTests)
    ----------------------------------------------------------------------
    Traceback (most recent call last):
      File "test\test_autopep8.py", line 4384, in test_recursive
        '\n'.join(result.split('\n')[3:5]))
    AssertionError: u"-'abc'  \n+'abc'" != u"-'abc'  \r\n+'abc'\r"
    - -'abc'
    + -'abc'
    ?         +
    ?       + +'abc'


    ----------------------------------------------------------------------
    Ran 472 tests in 23.736s

    FAILED (failures=5)