Handle `arc diff --background 1 --`
Handle `arc diff --background 1 --`

Summary: We currently insert background parameters to end which causes ignoring them if there is '--'.

Test Plan:
  $ arc diff --background 1 -- HEAD^

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3497

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