Add a check for ambiguous merge strategies after the "history.immutable" behavioral change

Summary: Ref T13546. When users hit a situation where we would select "squash" but would previously select "merge", prevent them from continuing under ambiguous conditions.

Test Plan: Ran "arc land" in Git with "history.immutable" true, false, and not configured.

Maniphest Tasks: T13546

Differential Revision: https://secure.phabricator.com/D21332