fix multiple issues in index-pack

fix multiple issues in index-pack

Since commit 9441b61dc5, two issues affected correct behavior of
index-pack:

 1) The real_type of a delta object is the 'real_type' of its base, not
    the 'type' which can be a "delta type".  Consequence of this is a
    corrupted pack index file which only needs to be recreated with a
    good index-pack command ('git verify-pack' will flag those).

 2) The code sequence:

        result->data = patch_delta(get_base_data(base), base->obj->size,
                                   delta_data, delta_size, &result->size);

    has two issues of its own since base->obj->size should instead be
    base->size as we want the size of the actual object data and not
    the size of the delta object it is represented by.  Except that
    simply replacing base->obj->size with base->size won't make the
    code more correct as the C language doesn't enforce a particular
    ordering for the evaluation of needed arguments for a function call,
    hence base->size could be pushed on the stack before get_base_data()
    which initializes base->size is called.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Tested-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>