Fix build_grouping_chain() to not clobber its input lists.

Fix build_grouping_chain() to not clobber its input lists.

There's no good reason for stomping on the input data; it makes the logic
in this function no simpler, in fact probably the reverse.  And it makes
it impossible to separate path generation from plan generation, as I'm
working towards doing; that will require more than one traversal of these
lists.