The index::nsec::staging_maintains_other_nanos test was created to
ensure that when we stage an entry when GIT_USE_NSECS is *unset* that
we truncate the index entry and do not persist the (old, invalid)
nanosec values.  Ensure that when GIT_USE_NSECS is *set* that we do
not do that, and actually write the correct nanosecond values.