rebase --autostash: demonstrate a problem with dirty submodules

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: https://github.com/git-for-windows/git/issues/1820

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