specs: what specific blocks should firehose "commit" blocks contain?

[bnewbold]

Proposed updated language:

The CAR file diff should contain blocks for all records mentioned in the ops list which are "created" or "updated", even if those blocks previously existed in the repo. Blocks for "deleted" records should not appear, unless there is another record created or updated with the same block CID in the same commit. The full "proof" chain of MST nodes for all the records relevant to the commit (created, updated, or deleted) should be included, from the chain all the way to the top of the tree, even if those MST nodes have appeared in previous commits (eg, if the repo is returning to a previous state).

See also: snarfed/bridgy-fed#1016