[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