Add a "--no-amend" flag to "arc diff"
Add a "--no-amend" flag to "arc diff"

Summary: This flag blocks amends.

Test Plan: Ran "arc diff --no-amend" ooooooh meta

Reviewers: davidreuss, btrahan

Reviewed By: davidreuss

CC: aran, epriestley

Maniphest Tasks: T822

Differential Revision: https://secure.phabricator.com/D1539

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