[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.