A repository's tree and blob pages name their phases in Server-Timing: session, git, highlight, render #3194

Browsing code is the commonest signed-out page after home. Its time is named so a slow tree can be traced.

Done looks like:

• [ ] The tree and blob pages (src/web/pages/browse.ts) send server-timing with git (the object reads), highlight (blob only) and render, as well as session.
• [ ] A test reads both headers from a fixture repository.

Files: src/web/pages/browse.ts, a test.