Make "arc diff --raw -C HEAD" work correctly
Make "arc diff --raw -C HEAD" work correctly

Summary:
There's no reason this can't work now, we're just not quite careful enough about definitions.

NOTE: If you use a message which has a revision ID in it already, this will fail in an obtuse way when launching the update editor fails because stdin isn't a tty. I'll clean this all up after T614 is fully sorted.

Test Plan: Ran "arc diff --raw -C HEAD".

Reviewers: btrahan, yairlivne

Reviewed By: btrahan

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1765

PageUpPeopleOrg/arcanist-patches,folsom-labs/arcanist,phacility/arcanist,folsom-labs/arcanist,coursera/arcanist,julkiewicz/arcanist,zhihu/arcanist,r4nt/arcanist,kevinseelbach/arcanist,memsql/arcanist,dguo-coursera/arcanist,wharris/arcanist,aik099/arcanist,hach-que/arcanist,Symplicity/arcanist,phacility/arcanist,wxstars/arcanist,alexmv/arcanist,KieranHayes/arcanist,jra3/arcanist,chundongwang/arcanist,uber/arcanist,KieranHayes/arcanist,alexmv/arcanist,phacility/arcanist,chundongwang/arcanist,codelabs-ch/arcanist,Khan/arcanist,aik099/arcanist,sneakpeeq/arcanist,fivestars/arcanist,r4nt/arcanist,aik099/arcanist,zhihu/arcanist,uber/arcanist,r4nt/arcanist,sneakpeeq/arcanist,phacility/arcanist,wikimedia/phabricator-arcanist,memsql/arcanist,fivestars/arcanist,chundongwang/arcanist,haskell-infra/arcanist,eSpark/arcanist,sneakpeeq/arcanist,calblueprint/arcanist,PageUpPeopleOrg/arcanist-patches,codelabs-ch/arcanist,wxstars/arcanist,fivestars/arcanist,memsql/arcanist,eSpark/arcanist,chundongwang/arcanist,alexmv/arcanist,julkiewicz/arcanist,wikimedia/phabricator-arcanist,dguo-coursera/arcanist,jra3/arcanist,codelabs-ch/arcanist,dguo-coursera/arcanist,codelabs-ch/arcanist,PageUpPeopleOrg/arcanist-patches,kevinseelbach/arcanist,uber/arcanist,alexmv/arcanist,hach-que/arcanist,KieranHayes/arcanist,hach-que/arcanist,wharris/arcanist,sneakpeeq/arcanist,folsom-labs/arcanist,aik099/arcanist,uber/arcanist,Khan/arcanist,Symplicity/arcanist,PageUpPeopleOrg/arcanist-patches,Symplicity/arcanist,Symplicity/arcanist,wikimedia/phabricator-arcanist,zhihu/arcanist,hach-que/arcanist,coursera/arcanist,julkiewicz/arcanist,folsom-labs/arcanist,haskell-infra/arcanist,haskell-infra/arcanist,Khan/arcanist,kevinseelbach/arcanist,Khan/arcanist,wharris/arcanist,coursera/arcanist,wxstars/arcanist,phacility/arcanist,fivestars/arcanist,kevinseelbach/arcanist,eSpark/arcanist,calblueprint/arcanist,wharris/arcanist,wxstars/arcanist,wikimedia/phabricator-arcanist,calblueprint/arcanist,jra3/arcanist,zhihu/arcanist,codelabs-ch/arcanist,KieranHayes/arcanist,calblueprint/arcanist,memsql/arcanist,r4nt/arcanist,jra3/arcanist,coursera/arcanist,julkiewicz/arcanist,dguo-coursera/arcanist,haskell-infra/arcanist,eSpark/arcanist