patcht2015-checkout-unborn.sh: changes the style for cd

> From: Ashutosh Pandey <ashutosh.pandeyhlr007@gmail.com>
>
> 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>
> ---
> this is my first time contributing to git I want to participate in 
> this year's GSoC and I'm looking for some microprojects the ones
> that are listed on the website are mostly done so if anyone can 
> suggest some beginners level microprojects that would be great.
>
>  t/t2015-checkout-unborn.sh | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/t/t2015-checkout-unborn.sh b/t/t2015-checkout-unborn.sh
> index 9425aae639..fdc4149f15 100755
> --- a/t/t2015-checkout-unborn.sh
> +++ b/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
>  '

> +	(
> +		cd parent &&
> +	 	git init &&
> +	 	echo content >file &&
> +	 	git add file &&
> +	 	git commit -m base