When updating revisions in response to commits, reuse previously generated diffs

Summary:
Fixes T10968. In rare situations, we can generate a diff, then hit an error which causes this update to fail.

When it does, we tend to get stuck in a loop creating diffs, which can fill the database up with garbage. We saw this once in the Phacility cluster, and one instance hit it, too.

Instead: when we create a diff, keep track of which commit we generated it from. The next time through, reuse it if we already built it.

Test Plan:

• Used bin/differential attach-commit <commit> <revision> to hit this code.
• Simulated a filesystem write failure, saw the diff get reused.
• Also did a normal update, which worked properly.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10968

Differential Revision: https://secure.phabricator.com/D17164