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/tui/theme/issues.go

@@ -41,6 +41,8 @@ func RoleWord(r submodule.Role) string {
 		return "HEAD"
 	case submodule.RoleStaged:
 		return Staged
+	case submodule.RoleBase:
+		return "base"
 	case submodule.RoleTip:
 		return "tip"
 	default: