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:

internal/tui/theme/theme.go

@@ -54,17 +54,17 @@ func Fg(c lipgloss.TerminalColor) lipgloss.Style {
 
 // Glyphs are the few symbols used next to the state words.
 type Glyphs struct {
-	Staged, Ahead, Behind, Collapsed, Expanded, Problems, Minus, From, Rule, Node, Gap string
+	Staged, Ahead, Behind, Collapsed, Expanded, Problems, Minus, From, Rule, Node, Gap, Join string
 }
 
 func Unicode() Glyphs {
 	return Glyphs{
-		Staged: "●", Ahead: "↑", Behind: "↓", Collapsed: "▸", Expanded: "▾", Problems: "·", Minus: "−", From: "←", Rule: "─", Node: "●", Gap: "┊",
+		Staged: "●", Ahead: "↑", Behind: "↓", Collapsed: "▸", Expanded: "▾", Problems: "·", Minus: "−", From: "←", Rule: "─", Node: "●", Gap: "┊", Join: "╯",
 	}
 }
 
 func ASCII() Glyphs {
 	return Glyphs{
-		Staged: "*", Ahead: "^", Behind: "v", Collapsed: ">", Expanded: "v", Problems: ".", Minus: "-", From: "<-", Rule: "-", Node: "o", Gap: ":",
+		Staged: "*", Ahead: "^", Behind: "v", Collapsed: ">", Expanded: "v", Problems: ".", Minus: "-", From: "<-", Rule: "-", Node: "o", Gap: ":", Join: "/",
 	}
 }