Sanitize QGitRepository cleanup semantics

Before, various things would cause the pointer to become dangling
and git_repository_free to be called multiple times, such as:

* Using the copy constructor
* Using the functions that return a repository reference,
like QGitObject::owner()

Now we use a smart pointer to handle the cleanup, and unless we
explicitly tell it to, QGitRepository will not take ownership
of a pointer passed to the constructor.