Skip completed display JSON

[yifanmai]

This significantly speeds up helm-summarize for the use case where we run helm-summarize on a run suite with some existing runs, add some runs using helm-run, and then run helm-summarize over the suite again. In this case, we skip re-computing the display JSON for the existing runs.

[percyliang]

Looks good, but also hard for me to reason globally about the flow (when this flag should be used). Is the whole process (especially for making incremental changes) documented somewhere?

[yifanmai]

There are still likely changes to the workflow in the near future, with helm-summarize changes, so maybe we add docs after that.