R1:6eae75d8f2ff

When building a synthetic diff in Differential, adjust diff properties correctly

Summary:
See PHI1218. When rendering "A vs B", we currently show the properties of diff A without modification.

Instead, take properties from the same place we're taking change details.

See T12664 for a followup.

Test Plan:
- In diff A, removed "+x" from a file.
- In diff B, changed the file but did not remove "+x" from it.
- Diffed B vs A.
- Before change: UI incorrectly shows "+x" removed (both sides incorrect…