We're using essentially the same logic to setup Git for committing in
multiple places but the way we're doing so is inconsistent. Moved to
using two shared utility methods and use them consistently.

This naming has always been a bit weird and confusing so let's change
it and provide a `compat` fallback for later deprecation.

This means the output is consistent and ensures that we get rid of any local cruft when installing.

This provides a more consistent version for `rubocop` than relying on
`Homebrew.install_gem_setup_path!` (and we really want `brew style` to
provide consistent output).

Newer versions require the systems RubyGems to be updated.

This simplifies the process of vendoring gems given `Gemfile` or
`Gemfile.lock` changes.