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 attached to email. Set the byte limit very low, saw patches get thrown away.

Reviewers: chad, amckinley

Reviewed By: amckinley

Maniphest Tasks: T12033

Differential Revision: https://secure.phabricator.com/D18598