No more lightgreen, it's unreadable in solarized color scheme

People complains lightgreen is nearly unreadable in solarized dark color
scheme.  Use green instead, which looks good enough for both

cdiff.py

         return colorize(line, 'lightred')
 
     def _markup_new(self, line):
-        return colorize(line, 'lightgreen')
+        return colorize(line, 'green')
 
     def _markup_mix(self, line, base_color):
         del_code = COLORS['reverse'] + COLORS[base_color]