Fix `arc diff --raw`
Fix `arc diff --raw`

Summary: I broke this in D3750. Calling `getRepositoryAPI()` triggers a check for workflow requirement of the repository API. Instead, we should just check if we alreayd have one.

Test Plan: Ran `cat x | arc diff --raw`.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1998

Differential Revision: https://secure.phabricator.com/D3848

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