Usage update for --width=0

README.rst

       --version           show program's version number and exit
       -h, --help          show this help message and exit
       -s, --side-by-side  enable side-by-side mode
-      -w N, --width=N     set text width for side-by-side mode, default is 80
+      -w N, --width=N     set text width for side-by-side mode, 0 for auto
+                          detection, default is 80
       -l, --log           show log with changes from revision control
       -c M, --color=M     colorize mode 'auto' (default), 'always', or 'never'
 

cdiff.py

         help='enable side-by-side mode')
     parser.add_option(
         '-w', '--width', type='int', default=80, metavar='N',
-        help='set text width for side-by-side mode. If set to "0", cdiff ' \
-             'will attempt to fit the terminal width.')
+        help='set text width for side-by-side mode, 0 for auto detection, '
+             'default is 80')
     parser.add_option(
         '-l', '--log', action='store_true',
         help='show log with changes from revision control')