submodule.c: add .gitmodules staging helper functions

submodule.c: add .gitmodules staging helper functions

Add the new is_staging_gitmodules_ok() and stage_updated_gitmodules()
functions to submodule.c. The first makes it possible for call sites to
see if the .gitmodules file did contain any unstaged modifications they
would accidentally stage in addition to those they intend to stage
themselves. The second function stages all modifications to the
.gitmodules file, both will be used by subsequent patches for the mv
and rm commands.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>