Make the .git/safe directory in bin/setup

Our expected way of managing Rubies is with rbenv:

https://github.com/thoughtbot/laptop/blob/master/common-components/ruby-environment

We load rbenv and add `.git/safe/../../bin:$PATH` to our $PATH in:

https://github.com/thoughtbot/dotfiles/blob/master/zshrc

Loading rbenv in `zshrc` is recommended by the rbenv docs:

https://github.com/sstephenson/rbenv#basic-github-checkout

Assuming the binstubs for a project are in the local bin/ directory, we go a
step further to add the directory to shell $PATH so that rspec can be invoked
without the bin/ prefix:

<code>export PATH="./bin:$PATH"</code>

Doing so on a system that other people have write access to (such as a shared
host) is a security risk:

sstephenson/rbenv#309

The `.git/safe` convention addresses the security problem:

https://twitter.com/tpope/status/165631968996900865

This zsh fix may be necessary for OS users in order to fix a bug:

https://github.com/thoughtbot/laptop/blob/master/mac-components/zsh-fix