convert extension: Add support for username mapping
Allows mapping usernames to new ones during conversion process.
- Use -A option for first import
- Then at the end of the conversion process and if the destination
repo supports authorfile attribute, author map content is copied
to the file pointed by the authorfile call.
- On incremental conversions w/o any -A option specified, the
destination authorfile, if any, gets read automatically.
EG: This allows mapping unix system usernames used in CVS accounts
to a more typical "Firstname Lastname <address@server.org>" pair.