mirroring script: Improve default behavior

• Retrieve the list of urls already set, if github is already set, consider the mirror done and stops.

• Since all repositories are not configured to have a shortname, extract a 'display' uri to extract a correct shortname for the repository (ex: swh-indexer, swh-loader-git, etc...). This will permit to mirror with the same shortname in github our forge repositories. Also this removes the shortname parameter of the script.

Ideally, it'd be awesome if the other information could be retrieved
as well:

• repo's uri: there are at least 3 different uris in the forge ('display', 'raw', etc... which one do i choose?)
• repo's description (did not find it yet in the api call output),
• etc...

This script would only then have one parameter as input (at the
moment, the callsign).