message: added comment for read method to git indexer (reference implementation)
diff: `updateContent` should call `ops.save` and/or `ops.delete` as many
  times as necessary, then return a new `meta` state.
  + `read` should return an up-to-date json-api document directly from
  + the data source. It will sometimes be used to update caches.
  +
  - the Hub ensures that only one indexer per data source is
  instantiated and running at a given time. You don't need to
  implement locking in the indexer.
  