Merge branch 'sb/submodule-recursive-checkout-detach-head'

"git checkout --recursive" may overwrite and rewind the history of
the branch that happens to be checked out in submodule
repositories, which might not be desirable.  Detach the HEAD but
still allow the recursive checkout to succeed in such a case.

* sb/submodule-recursive-checkout-detach-head:
Documentation/checkout: clarify submodule HEADs to be detached
recursive submodules: detach HEAD from new state

diff --cc Documentation/git-checkout.txt
Simple merge

diff --cc submodule.c
Simple merge

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