fixup! checkout.c: enable fscache for checkout again

Back when this commit was crafted, parallel checkout did not exist yet.
But now it does. And it needs the same handling in the sequential case
(i.e. when it is not actually parallel because of not reaching the
threshold to actually need to fan out).

This fixes https://github.com/git-for-windows/git/issues/3904

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

parallel-checkout.c

@@ -633,6 +633,7 @@ static void write_items_sequentially(struct checkout *state)
 {
 	size_t i;
 
+	flush_fscache();
 	for (i = 0; i < parallel_checkout.nr; i++) {
 		struct parallel_checkout_item *pc_item = &parallel_checkout.items[i];
 		write_pc_item(pc_item, state);