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>

internal/submodule/mock/mock.go

@@ -278,6 +278,7 @@ func fixture() []node {
 		}},
 		{relPath: "libs/http", status: &submodule.Status{
 			Pin:        submodule.PinInfo{Ref: "origin/develop", Ahead: 1, Behind: 2, Pushed: true, Source: submodule.RefFromContains},
+			Bases:      submodule.Bases{Tip: commit("a9b8c7d", "Release 1.9.0", "frank", 20*24*time.Hour)},
 			Pinned:     httpPin,
 			CheckedOut: httpPin,
 			Tip:        httpTip,