Bug 931310: Further improve New Relic deploy ping.

* Move revision.txt handling to update instead of pre_update.
* Get a real change log instead of just a github url.
* Steal wonderful description code from chief-james (willkg)
Merge pull request #1449 from pmclanahan/improve-newrelic-deploy-ping-931310

Bug 931310: Further improve New Relic deploy ping.