built-in rebase: demonstrate regression with --autostash

An unnamed colleague of Ævar Arnfjörð Bjarmason reported a breakage
where a `pull --rebase` (which did not really need to do anything but
stash, see that nothing was changed, and apply the stash again) also
detached the HEAD.

This patch adds a minimal reproducer for this regression.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3420-rebase-autostash.sh

@@ -361,4 +361,12 @@ test_expect_success 'autostash with dirty submodules' '
 	git rebase -i --autostash HEAD
 '
 
+test_expect_failure 'branch is left alone when possible' '
+	git checkout -b unchanged-branch &&
+	echo changed >file0 &&
+	git rebase --autostash unchanged-branch &&
+	test changed = "$(cat file0)" &&
+	test unchanged-branch = "$(git rev-parse --abbrev-ref HEAD)"
+'
+
 test_done