repo: implement git_repository_commondir

[st3iny]

Hey folks, thanks for maintaining bindings to libgit2.

I implemented git_repository_commondir because I'm currently using it in a side project.

I'm an open source maintainer/developer myself so I know the game. Critical and honest feedback is always welcome.

[ehuss]

Thanks! I pushed a small change to include the short-line for the documentation. Usually the first line should be a short description of what the function does.