[PATCH 0/6] packed deltas with offset to base instead of sha1

The following patches are the result of my work to allow deltas to have their base specified as an offset within a pack instead of a 20 byte sha1.

This turned to be somewhat more involved than I originally expected. Given the relative pack reduction resulting from that, I might have decided against doing this if I had known beforehand how much work was needed.

But since it is done now I think it is worth merging nevertheless. Some parts are still pure code cleanups anyway.

Overall the larger projects are likely to benefit more as they have a larger proportion of deltas. On the historic Linux archive the pack reduction is about 5%.

Nicolas