Correct some references to '--diff-only', now '--preview'.
Correct some references to '--diff-only', now '--preview'.

Summary:

Test Plan:

Reviewers:

CC:

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