Fix potential state mismatch in graph overview

[mr-remington]

Fixes #327

This change fixes an issue where when a child stage completes, its status is incorrectly displayed on the parent stage in the graph overview.

This bug is particularly noticeable in the complexSmokes pipeline. While this approach seems reasonable given the available node data, it may not be the definitive solution. A second review would be helpful to confirm its correctness.

(In this before screen cap the state is that of Branch A, which is incorrect, we overwrite the state to that of topStage)
Before:

After:

Testing done

Ran this change through the tests nestedStages, skippedParallel, syntheticStages, and complexSmokes, noticed no adverse effects.

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[timja]

Seems to work fine based on testing various states using complex smokes:

1. slow pending branch, other branches with unstable and one with error, the error is shown.
2. slow pending branch, other one with unstable