Refine "arc land" behavior when pushing "onto" a new branch

Summary:
Ref T13546. If the "onto" branch doesn't exist yet and has a "/" in it, we need to preface it with "refs/heads" explicitly.

Fix a "false/null" issue with argument validation.

Possibly, "arc land" should prompt you before creating branches in the remote.

Test Plan: Ran "arc land --onto does-not-exist/1.1" and created a branch.

Maniphest Tasks: T13546

Differential Revision: https://secure.phabricator.com/D21341