Re: [PATCH v4 0/8] refs: add reflog support to `git refs migrate`

> The `git refs migrate` command was introduced in
> 25a0023f28 (builtin/refs: new command to migrate ref storage formats,
> 2024-06-06) to support migrating from one reference backend to another.
>
> One limitation of the feature was that it didn't support migrating
> repositories which contained reflogs. This isn't a requirement on the
> server side as repositories are stored as bare repositories (which do
> not contain any reflogs). Clients however generally use reflogs and
> until now couldn't use the `git refs migrate` command to migrate their
> repositories to the new reftable format.
>
> One of the issues for adding reflog support is that the ref transactions
> don't support reflogs additions:
>   1. While there is REF_LOG_ONLY flag, there is no function to utilize
>   the flag and add reflogs.
>   2. reference backends generally sort the updates by the refname. This
>   wouldn't work for reflogs which need to ensure that they maintain the
>   order of creation.
>   3. In the files backend, reflog entries are added by obtaining locks
>   on the refs themselves. This means each update in the transaction, will
>   obtain a ref_lock. This paradigm fails to accompany the fact that there
>   could be multiple reflog updates for a refname in a single transaction.
>   4. The backends check for duplicate entries, which doesn't make sense
>   in the context of adding multiple reflogs for a given refname.
>
> We overcome these issue we make the following changes:
>   - Update the ref_update structure to also include the committer
>   information. Using this, we can add a new function which only adds
>   reflog updates to the transaction.