[Sentry] Add git revision to traces + Keep some stack locals

Summary:
Christine, Eben,

I fixed T254 and also created a custom sentry processor to keep the last n locals from the stack trace.

Test Plan: No test plan - can't reach gunks :(

Reviewers: spang, emfree

Reviewed By: emfree

Maniphest Tasks: T254

Differential Revision: https://review.inboxapp.com/D298