rebase --autostash: demonstrate a problem with dirty submodules

It has been reported that dirty submodules cause problems with the
built-in rebase when it is asked to autostash. The symptom is:

	fatal: Unexpected stash response: ''

This patch adds a regression test that demonstrates that bug.

Original report: git-for-windows#1820

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

t/t3420-rebase-autostash.sh

@@ -351,4 +351,14 @@ test_expect_success 'autostash is saved on editor failure with conflict' '
 	test_cmp expected file0
 '
 
+test_expect_failure 'autostash with dirty submodules' '
+	test_when_finished "git reset --hard && git checkout master" &&
+	git checkout -b with-submodule &&
+	git submodule add ./ sub &&
+	test_tick &&
+	git commit -m add-submodule &&
+	echo changed >sub/file0 &&
+	git rebase -i --autostash HEAD
+'
+
 test_done