Re: [PATCH 8/9] refs: implement logic to migrate between ref storage formats

> With the introduction of the new "reftable" backend, users may want to
> migrate repositories between the backends without having to recreate the
> whole repository. Add the logic to do so.
>
> The implementation is generic and works with arbitrary ref storage
> formats because we only use.

> It does have a few limitations though:
>
>   - We do not migrate repositories with worktrees, because worktrees
>     have separate ref storages. It makes the overall affair more complex
>     if we have to migrate multiple storages at once.
>
>   - We do not migrate reflogs, because we have no interfaces to write
>     many reflog entries.
>
>   - We do not lock the repository for concurrent access, and thus
>     concurrent writes may make use end up with weird in-between states.
>     There is no way to fully lock the "files" backend for writes due to
>     its format, and thus we punt on this topic altogether and defer to
>     the user to avoid those from happening.
>
> In other words, this version is a minimum viable product for migrating a
> repository's ref storage format. It works alright for bare repos, which
> typically have neither worktrees nor reflogs.

> But it will not work for
> many other repositories without some preparations. These limitations are
> not set into stone though, and ideally we will eventually address them
> over time.
>
> The logic is not yet used by anything, and thus there are no tests for
> it. Those will be added in the next commit.
>
> Signed-off-by: Patrick Steinhardt <ps@pks.im>