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

src/applications/diffusion/controller/DiffusionRepositoryCreateController.php

@@ -473,8 +473,10 @@ final class DiffusionRepositoryCreateController
         "| `svn://svn.example.net/svnroot/` |\n".
         "| `file:///local/path/to/svnroot/` |\n".
         "\n\n".
-        "Make sure you specify the root of the repository, not a ".
-        "subdirectory.");
+        "You **MUST** specify the root of the repository, not a ".
+        "subdirectory. (If you want to import only part of a Subversion ".
+        "repository, use the //Import Only// option at the end of this ".
+        "workflow.)");
     } else {
       throw new Exception("Unsupported VCS!");
     }