When creating an empty repository with `git init-db` we do not
check for error codes returned by `git_config_set` functions.
This may cause the user to end up with an inconsistent repository
without any indication for the user.
Fix this problem by dying early with an error message when we are
unable to write the configuration files to disk.
Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>