Give the "Code" tab in Diffusion more consistent (path-retaining) behavior

Summary:
Fixes T13270. In Diffusion, the "Code" tab is linked in a weird way that isn't consistent with the other tabs.

Particularly, if you navigate to x/y/z/ and toggle between the "Branches" and "History" tabs (or other tabs), you keep your path. If you click "Code", you lose your path.

Instead, retain the path, so you can navigate somewhere and then toggle to/from the "Code" tab to get different views of the same path.

Test Plan: Browed into a repository, clicked "History", clicked "Code", ended up back in the place I started.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13270

Differential Revision: https://secure.phabricator.com/D20323