t1600: fix racy index.skipHash test

The test 1600.6 can fail under --stress due to mtime collisions. Most of the tests include a removal of the index file to guarantee that the index is updated. However, the submodule test addded in ee1f0c242ef (read-cache: add index.skipHash config option, 2023-01-06) did not include this removal. Thus, on rare occasions, the test can fail because the index still has a non-null trailing hash, as detected by the helper added in da9acde14ed (test-lib-functions: add helper for trailing hash, 2023-01-06). By removing the submodule's index before the 'git -C sub add a' command, we guarantee that the index is rewritten with the new index.skipHash config option. Signed-off-by: Derrick Stolee <derrickstolee@github.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>