recursive submodules: detach HEAD from new state

recursive submodules: detach HEAD from new state

When a submodule is on a branch and in its superproject you run a
recursive checkout, the branch of the submodule is updated to what the
superproject checks out. This is very unexpected in the current model of
Git as e.g. 'submodule update' always detaches the submodule HEAD.

Despite having plans to have submodule HEADS not detached in the future,
the current behavior is really bad as it doesn't match user expectations
and it is not checking for loss of commits (only to be recovered via the
reflog).

Detach the HEAD unconditionally in the submodule when updating it.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>