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.
Test that nanoseconds are round-tripped correctly when we read
an index file that contains them. We should, however, ignore them
because we don't understand them, and any new entries in the index
should contain a `0` nsecs field, while existing preserving entries.
