TODO

- UI
    - Fetch drop bundles
        - git (from "dumb" mirrors)

    - Apply patches from foreign drops
    - Allow building drop state from foreign drop

    - Improve $EDITOR use
        - don't launch unless isatty
        - for metadata json, use ft=javascript and add help text
        - re-launch when edit error

    - List auto-branches
    - More topic traversals (DFS, BFS, ..)

    - Handle $PAGER, like git does?
    - Diff/apply patch?
    - Teardown?

- Internals
    - Cache topic subjects
    - Open repo eagerly at outermost command layer

      So we can return actual iterators from commands. Do we need a repo always,
      though?

    - Index bundle hash -> header meta

      For dynamic repacking

    - Upstream bindings to git_indexer_*

      So that we can resolve delta bases without committing to the odb
      immediately. This would allow to deduplicate packfile storage, and
      partition the odb as follows:

      - store the bundle pack and associated index in a directory separate from
        the repo objects (possibly scoped by drop)
      - add this directory as an alternate to the repo, preventing compaction
        initiated by git
      - strip the pack data from the bundle and replace with a pointer to the
        pack
      - snapshots now become compactions of this alternate objects store,
        controlled by `it`
      - when a snapshot is taken, dangling bundle (-headers) may either be
        pruned, resulting in a 404 from the HTTP server when the bundle is
        requested, or the pointer be replaced by a marker indicating that the
        bundle was made obsolete by the corresponding snapshot bundle. The HTTP
        server may then send a redirect to the snapshot instead.

- Protocol
    - Implement automerge topics

- HTTP
    - Accept checkpoints if authorised