Prevent infinite loop when using on-quit (#1856)

app.go

@@ -35,6 +35,7 @@ type app struct {
 	menuCompInd    int
 	selectionOut   []string
 	watch          *watch
+	quitting       bool
 }
 
 func newApp(ui *ui, nav *nav) *app {
@@ -66,6 +67,14 @@ func newApp(ui *ui, nav *nav) *app {
 }
 
 func (app *app) quit() {
+	// Using synchronous shell commands for `on-quit` can cause this to be
+	// called again, so a guard variable is introduced here to prevent an
+	// infinite loop.
+	if app.quitting {
+		return
+	}
+	app.quitting = true
+
 	onQuit(app)
 
 	if gOpts.history {