MODULES-1571 Add Git Facts

Add some tests for the facts, and refactor the facts a bit to make them easier to test :+1: 
Yeah, I thought about refactoring this into:

```ruby
when 'windows'
  null_path = 'nul'
else
  null_path = '/dev/null' 
end
git_exec_path_cmd = "git --exec-path #{null_path}"
```