R1:f9b3673fbbe1

When mail (like "!history" mail) has multiple comments, label them separately

Summary:
Depends on D19372. Ref T13124. See PHI505. Currently, if you `!history` a task with a lot of comments, you get output like this:

> alice added a comment.
> bailey added a comment.
> alice added a comment.
> alice added a comment.
>
> AAAA
>
> BBBB
>
> AAAA
>
> AAAA

This is impossible to read. Put the "alice added a comment." headers above the actual comments for comments after the first.

These types of mail messages…