Fix generation of "branch" URIs in Subversion repositories

Summary:
"Branch" really means "repository main screen, with some branch selected", so a branch isn't actually required since we can just take you to the default.

Fixes an issue where new crumbs would throw an exception in SVN repositories.

Test Plan: Browed an SVN repo.

Reviewers: btrahan, mbishopim3

Reviewed By: mbishopim3

CC: aran

Differential Revision: https://secure.phabricator.com/D7099