Redesign using `git config` with this gem

### Subject of the issue
The Git module has config an
d global_config methods that are available only when Git module is included. Refactored the code to remove duplication and to make it more explicit we are managing git config not the library setup.

### Your environment

* git 1.x

### Expected behaviour

Design is TBD and will be documented here

### See Also

See #398 and #400