Modify SampleGlobalStep memory key to merge traverser sets, keeping o…

[andreachild]

…nly traversers that have been through the most loops.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

@@                 Coverage Diff                  @@
##             global-repeat-poc    #3217   +/-   ##
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  Coverage                     ?   76.76%           
  Complexity                   ?    14904           
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  Files                        ?     1157           
  Lines                        ?    72391           
  Branches                     ?     8018           
====================================================
  Hits                         ?    55572           
  Misses                       ?    13819           
  Partials                     ?     3000           

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[Cole-Greer]

Note: I don't think it's necessary to add a nested loop requirement to this step, as in theory the only time this will be needed is if its inside a nested repeat step, in which case RepeatStep itself will already be providing the nested loop requirement. A similar argument would apply to any other steps which may be given "loop awareness" such as RangeGlobalStep. Note how even LoopsStep does not explicitly set a single loop or nested loop requirement.