The new option --ignore-submodules can now be used to ignore changes in
submodules.

Why?  Sometimes it is not interesting when a submodule changed.

For example, when reordering some commits in the superproject, a dirty
submodule is usually totally uninteresting.  So we will use this option
in git-rebase to test for a dirty working tree.
Signed-off-by: NJohannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>