Deprecation of `core.fsyncobjectfiles` is causing test failures

Git is about to deprecate core.fsyncobjectfiles in Git v2.36.0 in favor of a new core.fsync setting that allows admins to have more control over which specific files are synced. We need to migrate to the new setting.

Part of the migration to core.fsync is also changes I made in the context of git#97 (closed) to enable syncing of references. We should enable this new feature to fix repository corruption we see on hard-resets.