R1:2dfe79cfc713

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…