Add tests for `git_config_get_string_const()`, check whether it
dies printing the line number and the file name if a NULL
value is retrieved for the given key.
Signed-off-by: NTanay Abhra <tanayabh@gmail.com>
Reviewed-by: NMatthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>