Redirect output to console in background mode of `arc diff`
Redirect output to console in background mode of `arc diff`

Summary: See D3208.

Test Plan:
  $ arc diff --background 1

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3210

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