Add a test repository

This is mostly for Travis' benefit, since it does a shallow clone, and many of the tests expect early commits in node-gitteh's history.

The test repository is copied verbatim from [libgit2](https://github.com/libgit2/libgit2/tree/development/tests-clar/resources/testrepo.git), and I fixturized some of the assertions' expected values.