Fix a check when deciding to destroy the local branch after "arc land"
Fix a check when deciding to destroy the local branch after "arc land"

Summary: Fixes T9660. The behavior for this check wasn't quite right -- we want to check the "source ref" (what we're landing) against the "target onto" (the branch we're landing it onto).

Test Plan:
  - Landed from `master` (tracking origin/master). No delete.
  - Landed from `feature1` (tracking local/master). Delete.
  - Landed from `feature2` (tracking origin/master). Delete.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9660

Differential Revision: https://secure.phabricator.com/D14358

haskell-infra/arcanist,folsom-labs/arcanist,codelabs-ch/arcanist,memsql/arcanist,aik099/arcanist,fivestars/arcanist,phacility/arcanist,chundongwang/arcanist,alexmv/arcanist,coursera/arcanist,uber/arcanist,r4nt/arcanist,chundongwang/arcanist,haskell-infra/arcanist,fivestars/arcanist,jra3/arcanist,r4nt/arcanist,uber/arcanist,zhihu/arcanist,calblueprint/arcanist,uber/arcanist,folsom-labs/arcanist,calblueprint/arcanist,codelabs-ch/arcanist,fivestars/arcanist,zhihu/arcanist,calblueprint/arcanist,aik099/arcanist,kevinseelbach/arcanist,coursera/arcanist,phacility/arcanist,jra3/arcanist,jra3/arcanist,wikimedia/phabricator-arcanist,codelabs-ch/arcanist,phacility/arcanist,zhihu/arcanist,chundongwang/arcanist,folsom-labs/arcanist,calblueprint/arcanist,coursera/arcanist,Khan/arcanist,aik099/arcanist,alexmv/arcanist,memsql/arcanist,coursera/arcanist,Khan/arcanist,codelabs-ch/arcanist,wikimedia/phabricator-arcanist,phacility/arcanist,kevinseelbach/arcanist,memsql/arcanist,alexmv/arcanist,fivestars/arcanist,chundongwang/arcanist,kevinseelbach/arcanist,haskell-infra/arcanist,alexmv/arcanist,wikimedia/phabricator-arcanist,phacility/arcanist,Khan/arcanist,folsom-labs/arcanist,codelabs-ch/arcanist,kevinseelbach/arcanist,haskell-infra/arcanist,r4nt/arcanist,r4nt/arcanist,zhihu/arcanist,memsql/arcanist,uber/arcanist,wikimedia/phabricator-arcanist,jra3/arcanist,aik099/arcanist,Khan/arcanist