Generate diff between given two lists of warnings.

        @param oldWarnings: parsed old warnings
        @param newWarnings: parsed new warnings
        @return: a dict object of diff

def generateDiff(self, oldWarnings, newWarnings):
        """
        Generate diff between given two lists of warnings.

        @param oldWarnings: parsed old warnings
        @param newWarnings: parsed new warnings
        @return: a dict object of diff
        """
        diffWarnings = {}

        for modulename in newWarnings:
            diffInModule = (
                newWarnings[modulename] -
                oldWarnings.get(modulename, set()))
            if diffInModule:
                diffWarnings[modulename] = diffInModule

        return diffWarnings