Read the change list and its counts from the index

Read the change list and its counts from the index

The header, the change list and the release artifact list now come from
the derived rows instead of the metadata branch:

- The header's open-changes badge is one COUNT, so a page that only shows
  the badge no longer reads `ls-tree`, `batch-check` and `batch` on every
  view. That is the read that grew with the repository's issues and
  comments.
- The change list, its state chips and its approval counts are queries.
  An approval still only counts when it names the request's current head
  and base, and eligibility is still the ACL's decision, so a stale
  approval is shown as history exactly as before.
- The release page's artifact list is the indexed pointer set.
- A change page reads its one request rather than every request; the
  conversation page keeps reading git, so the record it renders is
  authoritative rather than a copy.

On the fixture (40 files, 30 issues, 30 changes), git calls per page:

  issues list    5 -> 2        change list   6 -> 3
  releases       6 -> 3        commits       7 -> 4
  overview      19 -> 6 (repeat)  code       12 -> 9
  change view   14 -> 12

The remaining calls are the one `rev-parse` the refresh needs, the branch
and tree reads the page itself shows, and the detail pages' own git read.