arc land: add --delete-remote, raise an error for landing a branch onto itself
arc land: add --delete-remote, raise an error for landing a branch onto itself

Summary:
  - Allow users to delete the remote copy of a feature branch as well as the local one, for workflows that push feature branches. We test if the remote exists before trying to delete it.
  - Raise a better warning when you misuse "arc land".
  - I also wrote some documentation about this, see next diff.

Test Plan:
  - Tried to land a branch onto itself.
  - Ran "arc land --delete-remote" on feature branches with and without remote feature branches.

Reviewers: aurelijus, btrahan

Reviewed By: aurelijus

CC: aran

Maniphest Tasks: T1204

Differential Revision: https://secure.phabricator.com/D2445

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