R1:846d625ed0eb

[differential] gmail-compatible emails

Summary:
Gmail ignores text inside of [square brackets] when deciding what to group
together. This diff does two things to create the right behavior for gmail:

1. put the verb text inside of [square brackets] so different verbs don't
break gmail threading.
2. Add the Diff ID to the email thread, so different diffs with the same name
don't group together.

Furthermore, to aid in distinguishing who is doing what when the from field
can't be spoofed, this diff…