Make Subversion URI construction more consistent

Summary:
Ref T2230. SVN has some weird rules about path construction. Particularly, if you're missing a "/" in the remote URI right now, the change parsing step doesn't build the right paths.

Instead, build the right paths more intelligently.

Test Plan: Added and executed unit tests. Imported an SVN repo.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, jpeffer

Maniphest Tasks: T2230

Differential Revision: https://secure.phabricator.com/D7590