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 ee1f0c2
(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 da9acde (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 <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: derrickstolee

t/t1600-index.sh

@@ -88,6 +88,7 @@ test_expect_success 'index.skipHash config option' '
 	git -c protocol.file.allow=always submodule add ./ sub &&
 	git config index.skipHash false &&
 	git -C sub config index.skipHash true &&
+	rm -f .git/modules/sub/index &&
 	>sub/file &&
 	git -C sub add a &&
 	test_trailing_hash .git/modules/sub/index >hash &&