'Redo the diff when the expression changes'

def refresh(self):

    if (self.diff_expr is not None):
        self.diff_arg = utils.shell_split(self.diff_expr)
    elif (self.b is None):
        self.diff_arg = [self.a]
    else:
        self.diff_arg = [self.a, self.b]
    self.refresh_filenames()
