Merge branch 'fp/submodule-checkout-mode'

"submodule.*.update=checkout", when propagated from .gitmodules to
.git/config, turned into a "submodule.*.update=none", which did not
make much sense.

* fp/submodule-checkout-mode:
git-submodule.sh: 'checkout' is a valid update mode

diff --cc git-submodule.sh
Simple merge