diff options: Introduce --ignore-submodules

diff options: Introduce --ignore-submodules

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: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>