annotated_commit: provide refs and description

Differentiate between the ref_name used to create an annotated_commit
(that can subsequently be used to look up the reference) and the
description that we resolved this with (which _cannot_ be looked up).

The description is used for things like reflogs (and may be a ref name,
and ID something that we revparsed to get here), while the ref name must
actually be a reference name, and is used for things like rebase to
return to the initial branch.