pub(crate) const SINCE: HeaderName;Expand description
What a getRepo answer says about the since it was asked for.
Absent when nothing was asked, Diff when the delta was answered, and
OutdatedCursor when it could not be and the whole repository was sent
instead. The two values are the firehose’s own vocabulary — see
didbot_serve::subscribe::OUTDATED_CURSOR — because a cursor this server
can no longer place is the same condition on either endpoint, and a
consumer that already switches on one name should not have to learn a
second.