Summary:
Add 'stop' verb to histedit. This is the version I submited to the ML
which got rejected because it doesn't preserve hashes. However as arc will
always modify the hash anyway that's save for us anyawy.
Test Plan: run tests, worked.
Reviewers: durham, sid0
Subscribers: akushner
Differential Revision: https://phabricator.fb.com/D1606075
