t2018: don't lose return code of git commands

Fix invocations of git commands so their exit codes are not lost
within non-assignment command substitutions.

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

t/t2018-checkout-branch.sh

@@ -41,8 +41,12 @@ do_checkout () {
 		test_must_fail git checkout $opts $exp_branch $exp_sha
 	else
 		git checkout $opts $exp_branch $exp_sha &&
-		test $exp_ref = $(git rev-parse --symbolic-full-name HEAD) &&
-		test $exp_sha = $(git rev-parse --verify HEAD)
+		echo "$exp_ref" >ref.expect &&
+		git rev-parse --symbolic-full-name HEAD >ref.actual &&
+		test_cmp ref.expect ref.actual &&
+		echo "$exp_sha" >sha.expect &&
+		git rev-parse --verify HEAD >sha.actual &&
+		test_cmp sha.expect sha.actual
 	fi
 }
 
@@ -162,7 +166,8 @@ test_expect_success 'checkout -B to a merge base' '
 '
 
 test_expect_success 'checkout -B to an existing branch from detached HEAD resets branch to HEAD' '
-	git checkout $(git rev-parse --verify HEAD) &&
+	head=$(git rev-parse --verify HEAD) &&
+	git checkout "$head" &&
 
 	do_checkout branch2 "" -B
 '