Inform about changes made between last revision and commit

Summary:
This adds a link to [Closed] e-mail if it detects some changes.
It compares added and removed lines with 3 lines context.
The subtle form of informing is permissive to false negatives and positives.
I have an e-mail filter for [Closed] e-mails so I wouldn't personally notice this change - we should probably promote this feature a little bit.

Test Plan:
Reparse a diff with a change after last update.
Reparse a diff without a change after last update.

Reviewers: jungejason, epriestley

Reviewed By: jungejason

CC: aran, Koolvin, btrahan

Maniphest Tasks: T201

Differential Revision: https://secure.phabricator.com/D2540