built-in rebase: demonstrate that ORIG_HEAD is not set correctly

The ORIG_HEAD pseudo ref is supposed to refer to the original,
pre-rebase state after a successful rebase. Let's add a regression test
to prove that this regressed: With GIT_TEST_REBASE_USE_BUILTIN=false,
this test case passes, with GIT_TEST_REBASE_USE_BUILTIN=true (or unset),
it fails.

Reported by Nazri Ramliy.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

t/t3400-rebase.sh

@@ -59,6 +59,14 @@ test_expect_success 'rebase against master' '
 	git rebase master
 '
 
+test_expect_failure 'rebase sets ORIG_HEAD to pre-rebase state' '
+	git checkout -b orig-head topic &&
+	pre="$(git rev-parse --verify HEAD)" &&
+	git rebase master &&
+	test_cmp_rev "$pre" ORIG_HEAD &&
+	! test_cmp_rev "$pre" HEAD
+'
+
 test_expect_success 'rebase, with <onto> and <upstream> specified as :/quuxery' '
 	test_when_finished "git branch -D torebase" &&
 	git checkout -b torebase my-topic-branch^ &&