Simplify `Proxy.git_commit_create()` implementation

Change the implementation of the method to rely on ligit2 `git_commit_create_from_oids()` **[function](https://github.com/libgit2/libgit2/blob/development/include/git2/sys/commit.h#L23-L42)**.

Beside reducing the complexity of the method, this would avoid libgit2 calls to retrieve the pointers to the `Tree` and the parent `Commit`s. As a bonus, this should also allow to get rid of `DisposableEnumerable`.