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.

Test that we can properly abort a rebase when it is initialized by a
revspec.  This ensures that we do not conflate revspecs and refnames.

Remove traces of `git_blob_create_fromchunks`

Allow creating copies of `git_reference` objects.

Rebase improvements with IDs

tag: ignore extra header fields

While no extra header fields are defined for tags, git accepts them by
ignoring them and continuing the search for the message. There are a few
tags like this in the wild which git parses just fine, so we should do
the same.

:zap: some warnings