submodule add: extend force flag to add existing repos

submodule add: extend force flag to add existing repos

Currently the force flag in `git submodule add` takes care of possibly
ignored files or when a name collision occurs.

However there is another situation where submodule add comes in handy:
When you already have a gitlink recorded, but no configuration was
done (i.e. no .gitmodules file nor any entry in .git/config) and you
want to generate these config entries. For this situation allow
`git submodule add` to proceed if there is already a submodule at the
given path in the index.

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