Implement updating an already cached git gem

Source::Git#cache already behaves correctly if the repo dir exists so
there is no need to check for it.
Implement DependencyResolver::GitSet#find_all
git_gem returns four objects, not three.
Hook git gems into RequestSet

Now git gems can be installed.  Full bundler mode is not complete, but I
was able to install a git gem from a gem.deps.rb.