rebase -i squashes submodule changes into unrelated commit

rebase -i squashes submodule changes into unrelated commit

Attempting to rebase three-commit series (two regular changes, followed by
one commit that changes what commit is bound for a submodule path) to
squash the first two results in a failure; not just the first two commits
squashed, but the change to the submodule is also included in the result.

This failure causes the subsequent step to "pick" the change that actually
changes the submodule to be applied, because there is no change left to be
applied.

Signed-off-by: Junio C Hamano <gitster@pobox.com>