Give "Track Only" repository detail proper getters/setters

Summary: Depends on D19856. Ref T13222. See D19829. Make access to "Track Only" slightly cleaner and more consistent..

Test Plan: Set, edited, and removed "Track Only" settings for a repository. Saw sensible persistence and display behaviors.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13222

Differential Revision: https://secure.phabricator.com/D19857