message: Fix index_v version check.
  The whole point is that when loading a cache from an earlier version,
  that cache may be missing new entries. So don't load older caches.
diff: @@ -708,9 +708,9 @@ impl<'a> SummariesCache<'a> {
  .get(..4)
  .ok_or_else(|| anyhow::anyhow!("cache expected 4 bytes for index version"))?;
  let index_v = u32::from_le_bytes(index_v_bytes.try_into().unwrap());
  - if index_v > INDEX_V_MAX {
  + if index_v != INDEX_V_MAX {
  bail!(
  - "index format version {} is greater than the newest version I know ({})",
  + "index format version {} doesn't match the version I know ({})",
  index_v,
  INDEX_V_MAX
  );
  