Re: [PATCH v4 09/12] sequencer: rewrite update-refs as user edits todo list

>
> From: Derrick Stolee <derrickstolee@github.com>
>
> An interactive rebase provides opportunities for the user to edit the
> todo list. The --update-refs option initializes the list with some
> 'update-ref <ref>' steps, but the user could add these manually.
> Further, the user could add or remove these steps during pauses in the
> interactive rebase.
>
> Add a new method, todo_list_filter_update_refs(), that scans a todo_list
> and compares it to the stored update-refs file. There are two actions
> that can happen at this point:
>
> 1. If a '<ref>/<before>/<after>' triple in the update-refs file does not
>    have a matching 'update-ref <ref>' command in the todo-list _and_ the
>    <after> value is the null OID, then remove that triple. Here, the
>    user removed the 'update-ref <ref>' command before it was executed,
>    since if it was executed then the <after> value would store the
>    commit at that position.
>
> 2. If a 'update-ref <ref>' command in the todo-list does not have a
>    matching '<ref>/<before>/<after>' triple in the update-refs file,
>    then insert a new one. Store the <before> value to be the current
>    OID pointed at by <ref>. This is handled inside of the
>    init_update_ref_record() helper method.
>
> We can test that this works by rewriting the todo-list several times in
> the course of a rebase. Check that each ref is locked or unlocked for
> updates after each todo-list update. We an also verify that the ref