NSManagedObjects can be save and fetched.

- A git_reference is maintained to the most recently saved git_tree.
- Default values are not saved. Instead they are restored in newValuesForObjectWithID:withContext:error:.
Adding LevelDB as a submodule.
A new Libgit₂ backend that stores objects in LevelDb.
Adding LZ₄ (v104).

http://lz4.googlecode.com/svn/trunk/

Copyright (C) 2011-2013, Yann Collet.
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Compressing LevelDB’s sstables with LZ₄.

The subset of the Snappy API used by LevelDB has been implemented in terms of LZ₄.
Snappy’s Varint functions are still used to encode the pre-compressed size.
Simplify Libgit₂ object data base path handling.