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.