allow updating a diff when saving instead of forcing a create

[ajtmccarty]

IFC-580

the current approach for saving a diff requires always saving the entire thing. if a diff is very big, this will take a long time.

this PR updates the cypher query that saves a diff to allow for updating a diff in place instead of only creating one. more changes are necessary, but this will soon allow us to only save the changed parts of a diff instead of the whole thing.

[codspeed-hq]

CodSpeed Performance Report

Merging #5617 will not alter performance

_{Comparing ajtm-01292025-allow-diff-update-save (7dacbd3) with stable (7569469)}

Summary

✅ 11 untouched benchmarks