Re-instate dry-run logic in git-unpack-objects

Re-instate dry-run logic in git-unpack-objects

It can no longer be as verbose, since it doesn't have a good way to
resolve deltas (now that it is purely streaming, it cannot seek around
to read the objects a delta is based on).

But it can check that the thing unpacks cleanly at least as far as pack
syntax goes - all the objects uncompress cleanly, and the pack has the
right final SHA1.