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 same changes.
• Viewed revision:
  • Saw the image renderered in the interdiff.
• Applied patch.
• Ran bin/differential rebuild-changesets ....
• Viewed revision:
  • Saw both changesets collapse as unchanged.

Maniphest Tasks: T13522

Differential Revision: https://secure.phabricator.com/D21174