Fix completions option handling

The completion script was not working because the `without-completions` option doesn't exist, so the block inside [this conditional will not run](https://github.com/boxen/puppet-git/blob/master/files/brews/git.rb#L69)

https://github.com/boxen/puppet-git/commit/6d1722c25da2252cf414c44d84ee180f7889d4d0 was pushed in 1.2.1 and 1.2.2 but since it is not on master branch the 1.2.3 tag doesn't include this commit.

This is a cherry pick of the above commit to fix the issues with the complations scripts.