Implements part of the single source of truth audit
fixes for the topics/git section.

The previous examples did work however they had a problem when the
bin/setup script installed a different bundler version than the one in
your Gemfile.lock.  It is safer to use `/bin/herokuish procfile exec` to
get access to all the environment already installed for your application
instead of installing bundler again.

Update documentation to reflect removal

Auto-DevOps: allow to disable rollout status check

See merge request gitlab-org/gitlab-ce!28130