pack cleanups: actually use deltas when building pack files, fix bugs in delta generation

This fixes a couple of bugs in pack generation (OFS_DELTA offsets were wrong, deltas of objects larger than 2^16 bytes were corrupt) and then turns on use of deltas in pack generation.

I dithered about turning on deltas in pack generation, but it seems like the right thing to do now that it works in all cases I can think of being worth testing, and it was the only easy way to actually test the new delta code against git.