R1:6fb3f857fb57

Stop the bleeding caused by attaching enormous patches to revision mail

Summary:
Ref T12033. This is a very narrow fix for this issue, but it should fix the major error: don't attach patches if they're bigger than the mail body limit (by default, 512KB).

Specifically, the logs from an install in T12033 show a 112MB patch being attached, and that's the biggest practical problem here.

I'll follow up on the tasks with more nuanced future work.

Test Plan: Enabled `differential.attach-patches`, saw a patch…