A merge waited the whole window even when nothing assembling shared anything
with it. It now joins only a batch it is related to (ADR 0304 §1, read by ADR
0306 §3: repository, a module the cut would move, a graph edge, a group head),
folding the newer into the oldest when it relates to two. A merge related to
none starts a batch whose window is closed at once (PlanBatch.CutAtOnce),
unless its delivery group may still have a member to merge (ADR 0306 §2).
The controller cannot see a group's unmerged members (that is mesh-delivery's
record), so a merge from any head still waits, in a batch of its own: the safe
error is to wait. Walks stay one at a time; a batch cut at once queues behind
an open walk. A stop's merges walked again join a related batch, or their own.
jschoubben
6196a4815e