One test proves the Fork tree test on the web: from a fork's page you see its parent and children, choose how it follows, and offer a change up in one action #7756

For: P3 Fork tree. Its test says: from any repository you see its parent and children as a tree, choose how this fork follows, and offer a change up in one action. test/follow-e2e.test.ts proves updates flowing down across forges, but nothing walks the three web steps a person takes on one forge.

Done looks like:

• [ ] A new test/fork-tree-web.test.ts boots one forge with an original repository, a fork by a second person, and a fork of that fork by a third.
• [ ] As the middle fork's owner, GET /:owner/:name (src/web/pages/repo.ts) shows the parent and the child as a tree (links to both, the lineage from src/core/following.ts), and the following page (src/web/pages/following.ts) offers the follow modes.
• [ ] Posting the following form changes the mode, and the page shows the new mode after the redirect, through a same-origin POST with the CSRF check.
• [ ] With one commit of its own, one POST from the offer page (src/web/pages/offer-upstream.ts) opens exactly one pull on the parent, and the parent's pulls list shows it.
• [ ] Where a step takes more than one action or is missing, fix that page and name it in the pull. Change no test that already passes.

Files: test/fork-tree-web.test.ts, and only if a step fails: src/web/pages/repo.ts, src/web/pages/following.ts or src/web/pages/offer-upstream.ts. Risks: git fixtures are slow, so build them once in beforeAll (see test/fork-detach.test.ts).