If we do this unconditionally, the user will get a warning message

  cvs add: manual/CVS already exists

in the update case. This patch should thus work better.

Btw, I prefer to not use this script, because it does not handle removed
doc pages yet. In these cases, it is advisable to augment the .symlinks
file, so that users who use old URLs don't get HTTP 404 errors. But the
script does not do this yet.