Add branch, tag info to Diffusion Headers

Summary: Improves overall UX of browsing Diffusion. Clarifies branch and tag when possible, changes 'home' to 'code', uses tabs in more locations. Fixes T12837

Test Plan: Review branchs, tags, git, hg, search, browse, history.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T12837

Differential Revision: https://secure.phabricator.com/D18434