test combinations of @{} syntax

Now that we have several different types of @{} syntax, it
is a good idea to test them together, which reveals some
failures.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1508-at-combinations.sh

@@ -0,0 +1,51 @@
+#!/bin/sh
+
+test_description='test various @{X} syntax combinations together'
+. ./test-lib.sh
+
+check() {
+test_expect_${3:-success} "$1 = $2" "
+	echo '$2' >expect &&
+	git log -1 --format=%s '$1' >actual &&
+	test_cmp expect actual
+"
+}
+nonsense() {
+test_expect_${2:-success} "$1 is nonsensical" "
+	test_must_fail git log -1 '$1'
+"
+}
+fail() {
+	"$@" failure
+}
+
+test_expect_success 'setup' '
+	test_commit master-one &&
+	test_commit master-two &&
+	git checkout -b upstream-branch &&
+	test_commit upstream-one &&
+	test_commit upstream-two &&
+	git checkout -b old-branch &&
+	test_commit old-one &&
+	test_commit old-two &&
+	git checkout -b new-branch &&
+	test_commit new-one &&
+	test_commit new-two &&
+	git config branch.old-branch.remote . &&
+	git config branch.old-branch.merge refs/heads/master &&
+	git config branch.new-branch.remote . &&
+	git config branch.new-branch.merge refs/heads/upstream-branch
+'
+
+check HEAD new-two
+check "@{1}" new-one
+check "@{-1}" old-two
+check "@{-1}@{1}" old-one
+check "@{u}" upstream-two
+check "@{u}@{1}" upstream-one
+fail check "@{-1}@{u}" master-two
+fail check "@{-1}@{u}@{1}" master-one
+fail nonsense "@{u}@{-1}"
+nonsense "@{1}@{u}"
+
+test_done