Ignore dirty submodule states during rebase and stash

Ignore dirty submodule states during rebase and stash

When rebasing or stashing, chances are that you do not care about
dirty submodules, since they are not updated by those actions anyway.
So ignore the submodules' states.

Note: the submodule states -- as committed in the superproject --
will still be stashed and rebased, it is _just_ the state of the
submodule in the working tree which is ignored.

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