Add missing trailing slashes to crumb links in Diffusion

Summary:
In most cases this just makes the URIs more consistent, but it's funky/breakish for SVN repositories which are only partially tracked.

See also T3915, and IRC.

Test Plan:
  - Browsed some repositories, verified URIs generated as expected, with trailing slashes for directories.
  - Verified nothing goofy happened in the extremes (like double slashes on the first crumb).

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7209