Implemented a lister to list the repos for a given CGit instance.

Closes T1659

Streamline production of new listers by aggressively moving core
functionality into progressively inherited (A->B->C) base classes
with the transport layer abstracted.
This should make common individual forge listers straightforward to
produce with minimal customization. Github and Bitbucket listers
can be used as examples of the indexing type.

Related T613