Reviewed by Shinichiro Hamaji.

        wdiff_text throws ScriptError because wdiff returns non-zero when files differ
        https://bugs.webkit.org/show_bug.cgi?id=38246

        wdiff returns 0 when files are the same, 1 when they differ.
        run_command by default raises ScriptError if the return code is non-zero.
        Fixed this by adding a custom error handler which only raises if the
        return code is not 1.

        I broke up the huge wdiff_text() method into little pieces
        for easier unit testing.  There is only one functional change here
        and that is the addition of the custom error handler.

        * Scripts/webkitpy/layout_tests/port/base.py:
        * Scripts/webkitpy/layout_tests/port/base_unittest.py:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@58395 268f45cc-cd09-0410-ab3c-d52691b4dbfc