git-apply --directory: make --root more similar to GNU diff

git-apply --directory: make --root more similar to GNU diff

Applying a patch in the directory that is different from what the patch
records is done with --directory option in GNU diff.  The --root option we
introduced previously does the same, and we can call it the same way to
give users more familiar feel.

Signed-off-by: Junio C Hamano <gitster@pobox.com>