[RELENG-50] Create git config for build user

The [`-q` option being used with the Omnibus's new git-cache feature](https://github.com/opscode/omnibus-ruby/commit/84060de75f0642e64963940bd63332f27ea3cc32) is not quieting warning messages on the Windows build slaves. The easy fix appears to ensure the build user has a sane git config.

The original commit in omnibus-ruby has details on the required git config: https://github.com/opscode/omnibus-ruby/commit/84060de75f0642e64963940bd63332f27ea3cc32