Add trailing slash to dirs in raw listing

Created by: rvantonder

Adds a trailing slash for dirs in the raw file repo listing. There's no indication in the current listings if an entry is a dir or file, and this changes gives (at least some) indication. Unfortunately we can't reflect "IsDir" directly in the URL (see https://github.com/sourcegraph/sourcegraph/pull/800#discussion_r231761557)

Before:

After:

Test plan: None. This is a simple change--no existing tests it seems. Give me a hint how to mock this if you have a good idea.