Move Git-specific "arc land" parsing of "--onto" and "--remote" into GitLandEngine

Summary: Ref T13434. Move some git-engine-specific handling of "arc land" arguments into the Git engine. This prepares to handle Perforce workflows.

Test Plan: Will "arc land" this change.

Maniphest Tasks: T13434

Differential Revision: https://secure.phabricator.com/D20867