Correct some minor "arc land" workflow issues in Mercurial

Summary: Ref T9948. Ref T13546. Clean up some minor behaviors to allow "arc land" to function in the simplest cases again. Also, do a capability test for "prune" rather than just falling back.

Test Plan: Ran "arc land <mark>" in Mercurial, got changes pushed.

Maniphest Tasks: T13546, T9948

Differential Revision: https://secure.phabricator.com/D21350