Re: [PATCH 09/13] refs: introduce an iterator interface

> Currently, the API for iterating over references is via a family of
> for_each_ref()-type functions that invoke a callback function for each
> selected reference. All of these eventually call do_for_each_ref(),
> which knows how to do one thing: iterate in parallel through two
> ref_caches, one for loose and one for packed refs, giving loose
> references precedence over packed refs. This is rather complicated code,
> and is quite specialized to the files backend. It also requires callers
> to encapsulate their work into a callback function, which often means
> that they have to define and use a "cb_data" struct to manage their
> context.
>
> The current design is already bursting at the seams, and will become
> even more awkward in the upcoming world of multiple reference storage
> backends:
>
> * Per-worktree vs. shared references are currently handled via a kludge
>   in git_path() rather than iterating over each part of the reference
>   namespace separately and merging the results. This kludge will cease
>   to work when we have multiple reference storage backends.