Keep Arcanist from mistaking paths that look like svn:externals as svn:externals

Summary: Fixes T3920. Added a slash to the path and external name so that "public" and "publicnotexternal" won't appear to be the same root.

Test Plan:
We've had this issue in one of our projects for some time, just ran into it again today. Ran the patched arc against the same directory structure and the troublesome file was added to the diff. Confirmed that files
modified in the "public" (svn external) folder are still caught as external modifications.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T3920

Differential Revision: https://secure.phabricator.com/D7226