The evaluation of the reposioty identity is rewritten to better handle cases
where there are child and sibling revisions that are active at the same time.
In particular, `fn Identity::action` is now free of any references to
`self.current`.
This is achieved through two main improvements. The first is that the `State`
of revisions changes to improve clarity:
1. `Active` and `Accepted` remain, and their meanings also remain the same.
2. `Stale` is removed entirely.
3. `Rejected` is improved to also contain `RejectedBy`, to keep track of the
reason for rejection.
4. `Redacted` is promoted to a state. Similarly, the reason for
redaction is tracked by `RedactedBy`.
The transition of a revision from `Active` to one of the other states now
influences siblings and children.
If a revision transitions to `Accepted`, then the sibling revisions can no
longer transition to `Accepted`. They are considered `Rejected` where the reason
is `Sibling`. This cascades: Children of siblings are `Rejected`
recursively, tracking `RejectedBy::Parent`.
Any children of the `Accepted` revision are also evaluated to see if they can be
similarly transitioned to the `Accepted` state; since they may have already been
voted on.
If a revision was rejected by a majority, then the it transitions to `Rejected`
with `RejectedBy::Vote`. Dually to acceptance, the children of this revision
are also rejected with the reason of `Ancestor`.
Finally, when the author of a revision redacts a revision, it transitions
to `Redacted`, tracking `RedactedBy::Author`, and its children are redacted
tracking `RedactedBy::Parent`.
The test is adjusted to `remove_delegate_concurrent` reflect that concurrently
proposed revisions are retained in the timeline and explicitly marked as rejected,
rather than being dropped entirely (as before).
d8c14222a2