git add: Add the "--ignore-missing" option for the dry run

git add: Add the "--ignore-missing" option for the dry run

Sometimes it is useful to know if a file or directory will be ignored
before it is added to the work tree. An example is "git submodule add",
where it would be really nice to be able to fail with an appropriate
error message before the submodule is cloned and checked out.

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