[PATCH/RFC] Add support for subversion dump format v3

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

Patches 1 to 4 add the required infrastructure to fast-import.
The primary feature is the addition of the cat-file command to
fast-import. This allows access to objects written to the
the current pack prior to a checkpoint and is critical to
retrieving full-texts to drive the diff applier.

Patch 5 adds the basic parsing necessary to process the v3 format.

Patch 6 adds logic around decoding prop deltas.

Patch 7 integrates svn-fe with svn-da to decode text deltas.
It was primarily authored by Jonathan but inspired by Ram and
completed by myself.
This is probably the most sprawling of the series and likely the
heaviest target of review.

 Documentation/git-fast-import.txt |   49 +++++++++
 contrib/svn-fe/svn-fe.txt         |    6 +-
 fast-import.c                     |  147 +++++++++++++++++++++++++-
 t/t9010-svn-fe.sh                 |    6 +-
 t/t9300-fast-import.sh            |  210 +++++++++++++++++++++++++++++++++++++
 test-svn-fe.c                     |    3 +-
 vcs-svn/fast_export.c             |  187 ++++++++++++++++++++++++++++++++-
 vcs-svn/fast_export.h             |   13 ++-
 vcs-svn/repo_tree.c               |   37 +++++++-
 vcs-svn/repo_tree.h               |    3 +
 vcs-svn/svndiff.c                 |   11 +--
 vcs-svn/svndiff.h                 |    3 +-
 vcs-svn/svndump.c                 |   99 ++++++++++++++++--
 13 files changed, 739 insertions(+), 35 deletions(-)