As git_annotated_commit seems to behave like cgit's refish, it's quite
helpful to abstract away "targets" via git_annotated_commit_from_id/from_ref.
As the former is accessible via git_annotated_commit_id, make the latter
also available to users.
This extra constructor will be useful for the annotated versions of
ref-modifying functions, as it allows us to create a commit with the
extended sha syntax which was used to retrieve it.
