[PATCHv2] Add support for subversion dump format v3

This series follows Jonathan Nieder's svn diff applier series.

Patch 1 adds the required infrastructure to fast-import. This features the addition of the cat-blob command to fast-import. This allows access to blobs written to the the current pack prior to a checkpoint and is critical to retrieving full-texts to drive the diff applier.

Patch 2 adds the basic parsing necessary to process the v3 format.

Patch 3 adds logic around decoding prop deltas.

Patch 5 integrates svn-fe with svn-da to decode text deltas. It is based on a large patch authored by Jonathan and inspired by Ram. It has been heavily trimmed to reduce code bloat and enabled me to determine the logic for the previous patches.

A bit shout-out to Jonathan Nieder and Ramkumar Ramachandra for their help in bring this series into existence.

I have tried to incorporate all the feedback on the list and over at #git-devel. I hope to impress.

-- David Barr.