Simply expose each repository's fast_import member

We'll want direct access anyhow.
Expose git_fast_import through a member function

It was just too hard to make the class definition look right and still
maintain the ordering needed for construction.
Remove unused declaration
Add a set of refs to each git_repository
Add missing value_type typedef

In case we ever want to use this with std::inserter
Formatting tweak
Start handling ruleset transitions