Integrate `incoming` retired deltas from another branch and emit a [`RegistryDelta::Merge`] joining the resulting tips, returning the new merge `Rev` (or `None` if `incoming` adds nothing). Applies each incoming op to the registry, then hands the set to [`History::merge`]. Incoming deltas must arrive in causal order.
(&mut self, incoming: impl IntoIterator<Item = Delta>)
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