Implement Blob, Tree and Lazy, add tests for Tree.
Lots of changes this commit. More in-depth:
* Added text fixtures from Git Python
* Add Git->getDir
* Add Git::$executeKwords (doesn't do anything yet, we need to merge upstream
changes)
* Implemented Git_Tree expansion in Git_Commit
* Make Git_Repo properties public, as they are not lazy-loaded
* Implement Git_Repo->tree()
* Implement GitHarness, for easy access to fixtures
* Setup Mocks
Signed-off-by: Edward Z. Yang <edwardzyang@thewritingpot.com>