Verify that SVN repository roots really are repository roots

Summary: Fixes T3238. Ref T4327. Although the instructions are fairly clear on this, it's easy to miss them. Make sure the root the user enters matches the real root.

Test Plan: Added unit tests. Used `bin/repository discover` to hit the check explicitly.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3238, T4327

Differential Revision: https://secure.phabricator.com/D8020