Re: [PATCH v2] rebase --update-refs: avoid unintended ref deletion

> In b3b1a21d1a5 (sequencer: rewrite update-refs as user edits todo list,
> 2022-07-19), the 'todo_list_filter_update_refs()' step was added to handle
> the removal of 'update-ref' lines from a 'rebase-todo'. Specifically, it
> removes potential ref updates from the "update refs state" if a ref does not
> have a corresponding 'update-ref' line.
> 
> However, because 'write_update_refs_state()' will not update the state if
> the 'refs_to_oids' list was empty, removing *all* 'update-ref' lines will
> result in the state remaining unchanged from how it was initialized (with
> all refs' "after" OID being null). Then, when the ref update is applied, all
> refs will be updated to null and consequently deleted.
> 
> To fix this, delete the 'update-refs' state file when 'refs_to_oids' is
> empty. Additionally, add a tests covering "all update-ref lines removed"
> cases.
> 
> Reported-by: herr.kaste <herr.kaste@gmail.com>
> Helped-by: Phillip Wood <phillip.wood@dunelm.org.uk>
> Helped-by: Derrick Stolee <derrickstolee@github.com>
> Signed-off-by: Victoria Dye <vdye@github.com>
> ---
> Changes since v1:
> - Modified approach to handling empty 'refs_to_oids' from "optional force write
>   empty file" to "always unlink"
> - Added/updated tests