p2000: explicitly turn off sparse index

The full index test cases in p2000-sparse-operations.sh relied on the
fact that the sparse index was off by default. When this default was
flipped, the performance test was not altered. Do so now with the
--no-sparse-index flag.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>

t/perf/p2000-sparse-operations.sh

@@ -55,7 +55,7 @@ test_expect_success 'setup repo and indexes' '
 	git -c core.sparseCheckoutCone=true clone --branch=wide --sparse . full-v3 &&
 	(
 		cd full-v3 &&
-		git sparse-checkout init --cone &&
+		git sparse-checkout init --cone --no-sparse-index &&
 		git sparse-checkout set $SPARSE_CONE &&
 		git config index.version 3 &&
 		git update-index --index-version=3 &&
@@ -64,7 +64,7 @@ test_expect_success 'setup repo and indexes' '
 	git -c core.sparseCheckoutCone=true clone --branch=wide --sparse . full-v4 &&
 	(
 		cd full-v4 &&
-		git sparse-checkout init --cone &&
+		git sparse-checkout init --cone --no-sparse-index &&
 		git sparse-checkout set $SPARSE_CONE &&
 		git config index.version 4 &&
 		git update-index --index-version=4 &&