fetchpack.c: replace `git_config()` with `git_config_get_*()` family

fetchpack.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.

Signed-off-by: Tanay Abhra <redacted>
Reviewed-by: Matthieu Moy <redacted>
Signed-off-by: Junio C Hamano <redacted>