Runs msgmerge on a locale_file and po_string

def msgmerge(self, locale_file, po_string):
        """
        Runs msgmerge on a locale_file and po_string
        """

        cmd = "msgmerge -q %s -" % locale_file
        p = subprocess.Popen(cmd, shell=True, stdin=subprocess.PIPE,
            stdout=subprocess.PIPE, stderr=subprocess.PIPE)
        (msg, err) = p.communicate(input=po_string)

        if err:
            # dont raise exception, some stuff in stderr are just warmings
            logging.warning("%s \nfile: %s\npostring: %s" % (err, locale_file, po_string))

        return msg