Index state changes for the activity feed

Index state changes for the activity feed

The activity feed found closed, reopened and merged events by reading the
last 200 metadata commit subjects on every render: one git process per
repository, so the instance home with N repositories ran N of them. It was
the last derived read still going to git.

- A `state_changes` table is fed by the same refresh: a delta reads the
  commits in its range, a rebuild reads the whole metadata branch. Rows are
  keyed by the commit, so a retried or overlapping refresh cannot duplicate
  an event, and an ordinal keeps two changes in one second in branch order
  (commit times are seconds; a close and a reopen often share one).
- `Store.StateChanges` returns the events newest first; the feed reads it
  and keeps the metadata log as the fallback for a server without an index
  or a failed read.
- `git.Repo.Log` and `CommitsBetween` treat a limit of zero as no limit,
  which the rebuild needs, and `metadata.ParseStateSubject` is exported so
  the index derives the events without a second copy of the format.

The activity page now starts no metadata log; the instance home and the
instance activity page lose one process per visible repository. On the
load harness with three repositories: activity 496 req/s (p50 7 ms),
instance activity 230 req/s (p50 17 ms), home 202 req/s (p50 20 ms).

Schema version 7 rebuilds each repository on its first read after upgrade.