increment_graph_window: don't revisit nodes

[oliver-sanders]

Avoid re-visiting nodes in the increment_graph_window algorithm.

This will help to reduce the number of recursive calls and will allow us to efficiently include "cousins" in the n>1 window.

Note: Cousins
For the graph a => <b> => c, if b1 is active, then b2 is a cousin which should be included in the n=2 window.

See this comment and the larger issue for discussion:

Not revisiting nodes

we can do this by storing graph relations in an easily referenced way (if we don't already)
adopting graph sections visited, and skipping walks to head of adopted section (if required)

--- #5435 (comment)

[MetRonnie]

Closed by #5600