Merge branch 'sz/submodule-force-update'

"git submodule update --force" used to leave the working tree of the
submodule intact when there were local changes.  It is more intiutive
to make "--force" a sign to run "checkout -f" to overwrite them.

* sz/submodule-force-update:
Make 'git submodule update --force' always check out submodules.

diff --cc git-submodule.sh
Simple merge

diff --cc t/t7406-submodule-update.sh
Simple merge