Draw diverged commits on the spine as a fork

A tip, checkout or committed pin that has diverged from the pin gets its
own lane, which joins the main lane at a "base" node: the merge base of
the two, with its SHA and subject. Only one diverged commit is drawn as a
fork; with two, or without a merge base, the nodes stay in a line with
the "diverged" gap.

Add Status.Bases, filled by the git provider with `merge-base` for each
diverged commit.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

docs/requirements.md

@@ -64,6 +64,8 @@ commit hashes. It doesn't answer the questions users actually have:
   **tip** and the checkout (**you**) as one line of history, newest on top:
   a node per commit (SHA + subject) with its roles, and the number of
   commits between two nodes. Roles on the same commit share a node. A
+  commit that has diverged from the pin is drawn as a fork that joins at
+  the commit where the histories split (`base`). A
   checkout that differs from the pin is highlighted.
 - **F22** *(dropped)* The commits between pin and tip are not listed; only
   their number is shown (F21).