update submodules: add submodule_move_head

update submodules: add submodule_move_head

In later patches we introduce the options and flag for commands
that modify the working directory, e.g. git-checkout.

This piece of code will be used universally for
all these working tree modifications as it
* supports dry run to answer the question:
  "Is it safe to change the submodule to this new state?"
  e.g. is it overwriting untracked files or are there local
  changes that would be overwritten?
* supports a force flag that can be used for resetting
  the tree.

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