t2015-checkout-unborn.sh: changes the style for cd

the `cd` followed the old style which wasn't consistent with the rest of
the test suite, so this commit makes it consistent with the current
style of the test suite for `cd` in  subshell.

Signed-off-by: Ashutosh Pandey <ashutosh.pandeyhlr007@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t2015-checkout-unborn.sh

@@ -9,11 +9,12 @@ TEST_PASSES_SANITIZE_LEAK=true
 
 test_expect_success 'setup' '
 	mkdir parent &&
-	(cd parent &&
-	 git init &&
-	 echo content >file &&
-	 git add file &&
-	 git commit -m base
+	(
+		cd parent &&
+		git init &&
+		echo content >file &&
+		git add file &&
+		git commit -m base
 	) &&
 	git fetch parent main:origin
 '