Support for Graphs

This PR adds ```Git::Raw::Graph```, with method ```is_ancestor_of```.

Also added:
* Internal method ```git_sv_to_commitish``` which can resolve a "commitish" to a commit id, that is a ```Git::Raw::Reference```, ```Git::Raw::Commit``` or an object id (or its prefix).
* ```Git::Raw::Commit->ancestor()```
* ```Git::Repository->merge_base()``` can also now accept a "commitish" object, like ```Git::Raw::Graph->is_descendant_of()```