Use Homebrew for Git

We previously relied on the version of Git
that comes with Xcode.
The Homebrew version is easier to upgrade
when new versions of Git are released,
such as yesterday's security patch:

https://github.com/blog/1938-git-client-vulnerability-announced
Use rbenv shell instead of rbenv local

`rbenv local` creates a `.ruby_version` file
in the current directory.