update copyright on modified tree

[Peter Johansson]

related to ticket:276

It would be useful if svndigest could update the copyright also on a modified WC. One could, for example, use that feature just prior checking in to the repo.

Since the beginning we require that WC is not modified. I don't remember exactly why, but it seems to make sense when parsing the history. However, in this case one could see a solution in which we don't parse the history. Either by require an uptodate cache or that copyright statements are just added (see ticket:276).