Expose the `config_set` C API as a set of simple commands in order to
facilitate testing. Add tests for the `config_set` API as well as for
`git_config_get_*()` family for the usual config files.
Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr> Signed-off-by: Tanay Abhra <tanayabh@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>