git-pack-objects: create a packed object representation.

git-pack-objects: create a packed object representation.

This is kind of like a tar-ball for a set of objects, ready to be
shipped off to another end.  Alternatively, you could use is as a packed
representation of the object database directly, if you changed
"read_sha1_file()" to read these kinds of packs.

The latter is partiularly useful to generate a "packed history", ie you
could pack up your old history efficiently, but still have it available
(at a performance hit, of course).

I haven't actually written an unpacker yet, so the end result has not
been verified in any way yet.  I obviously always write bug-free code,
so it just has to work, no?