Fix over-matching of quoted text for message bodies beginning with "On..."

Summary:
A user sent a message to Phabricator which looked like:

On blah blah blah ?

On <date>, <user> wrote:
> blah blah blah

The current algorithm is too aggressive and thinks lines 1-3 are all the "On ... wrote:" string. Instead, patch only the most recent "On".

Test Plan: Added a failing test and made it pass.

Reviewers: btrahan, zeeg

Reviewed By: zeeg

CC: aran

Differential Revision: https://secure.phabricator.com/D7732