Writing Clar tests for libgit2

For information on the Clar testing framework and a detailed introduction please visit:

https://github.com/vmg/clar

• Write your modules and tests. Use good, meaningful names.

• Make sure you actually build the tests by setting:

    cmake -DBUILD_CLAR=ON build/
  

• Test:

    ./build/libgit2_clar
  

• Make sure everything is fine.

• Send your pull request. That's it.