nsec: update staging test for GIT_USE_NSECS

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.

tests/index/nsec.c

@@ -61,8 +61,17 @@ void test_index_nsec__staging_maintains_other_nanos(void)
 	cl_assert_equal_b(true, has_nsecs());
 
 	cl_assert((entry = git_index_get_bypath(repo_index, "a.txt", 0)));
+
+	/* if we are writing nanoseconds to the index, expect them to be
+	 * nonzero.  if we are *not*, expect that we truncated the entry.
+	 */
+#ifdef GIT_USE_NSEC
+	cl_assert(entry->ctime.nanoseconds != 0);
+	cl_assert(entry->mtime.nanoseconds != 0);
+#else
 	cl_assert_equal_i(0, entry->ctime.nanoseconds);
 	cl_assert_equal_i(0, entry->mtime.nanoseconds);
+#endif
 }
 
 void test_index_nsec__status_doesnt_clear_nsecs(void)