Adding Libgit₂ as a submodule.

libgit2 is Copyright (C) the libgit2 contributors.

Note that the only valid version of the GPL as far as this project is concerned is _this_ particular version of the license (ie v2, not v2.2 or v3.x or whatever), unless explicitly otherwise stated.

LINKING EXCEPTION

In addition to the permissions in the GNU General Public License, the authors give you unlimited permission to link the compiled version of this library into combinations with other programs, and to distribute those combinations without any restriction coming from the use of this file. (The General Public License restrictions do apply in other respects; for example, they cover modification of the file, and distribution when not linked into a combined executable.)
Moving Libgit₂ to tag v0.18.0

This is the last minor release before 1.0preview1.

Highlights of this release include:

- Branch API
- Checkout head, index and tree
- Finished clone support
- Abstracted reference API to use custom backends
- Full diff support
- New (faster) packbuilder
- Push support
- New Remotes API
- Revparse support (single and range commits)
- Stash support
- Submodules support
A minimal application for hosting unit tests.
Creating a GitIncrementalStore.
All empty repositories should have an initial commit.