cgit lister: add option to exclude certain paths

For AFNR#502 they wanted to exclude non-redistributable and non-free projects from being archived by SWH. The current solution is that they are unlisted. The forge admins would like to make them publicly listed, under the "/unfree" path and have SWH exclude that path from our archiving activities.

Once the feature is implemented and the parameter is set for this forge, we should contact them to let them know that they can re-enable listing of the unfree repositories.