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