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, just showing the change from diff A).
  • After change: UI shows 100644 -> null, which is half right.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20478