read-cache.c: replace `git_config()` with `git_config_get_*()` family

read-cache.c: replace `git_config()` with `git_config_get_*()` family

Use `git_config_get_*()` family instead of `git_config()` to take
advantage of the config-set API which provides a cleaner control flow.

Use an intermediate value, as `version` can not be used directly in
git_config_get_int() due to incompatible type.

Signed-off-by: Tanay Abhra <tanayabh@gmail.com>
Reviewed-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>