R1:c12db282519d

For changesets that affect binaries, use the new binary file content hash as an effect hash

Summary: Ref T13522. When changesets update an image, we currently compute no effect hash. A content hash of the image (or other binary file) is a reasonable effect hash, and enalbes effect-hash-based behavior, including hiding files in intradiffs.

Test Plan:
- Created a revision affecting `cat.png` and `quack.txt` (currently, there must be 2+ changesets to trigger the hide logic).
- Updated it with the exact…