submodule deinit test: fix broken && chain in subshell

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7400-submodule-basic.sh

@@ -857,7 +857,7 @@ test_expect_success 'submodule deinit works on repository without submodules' '
 		git init &&
 		>file &&
 		git add file &&
-		git commit -m "repo should not be empty"
+		git commit -m "repo should not be empty" &&
 		git submodule deinit .
 	)
 '