Improve the logic for identifying ambiguous commits and applying "--revision" to them

Summary: Ref T13546. This is mostly minor cleanup that improves behavior under "--revision".

Test Plan: Ran arc land --into-empty and arc land --into-empty --revision 123 with ambiguous revisions in history to hit both the force and non-force outcomes.

Maniphest Tasks: T13546

Differential Revision: https://secure.phabricator.com/D21325