t2018: improve style of if-statement

Convert `[]` to `test` and break if-then into separate lines, both of
which bring the style in line with Git's coding guidelines.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t2018-checkout-branch.sh

@@ -20,7 +20,8 @@ do_checkout () {
 	exp_sha=${2:-$(git rev-parse --verify HEAD)} &&
 
 	# default options for git checkout: -b
-	if [ -z "$3" ]; then
+	if test -z "$3"
+	then
 		opts="-b"
 	else
 		opts="$3"