gc: perform incremental repack when implictly enabled

gc: perform incremental repack when implictly enabled

builtin/gc.c has two ways of checking if multi-pack-index is enabled:
- git_config_get_bool() in incremental_repack_auto_condition()
- the_repository->settings.core_multi_pack_index in
  maintenance_task_incremental_repack()

The two implementations have existed since the incremental-repack task
was introduced in e841a79a13 (maintenance: add incremental-repack auto
condition, 2020-09-25). These two values can diverge because
prepare_repo_settings() enables the feature in the_repository->settings
by default.

In the case where core.multiPackIndex is not set in the config, the auto
condition would fail, causing the incremental-repack task to not be
run. Because we always want to consider the default values, we should
always use the_repository->settings.

Standardize on using the_repository->settings.core_multi_pack_index to
check if multi-pack-index is enabled.

Signed-off-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>