Shorten excuse prompt in `arc diff`
Shorten excuse prompt in `arc diff`

Test Plan: `arc diff` on a commit with lint error.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Koolvin

Differential Revision: https://secure.phabricator.com/D2595

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