Set groundwork for better disabled reasons with range select

Something dumb that we're currently doing is expecting list items
to define an ID method which returns a string. We use that when copying
items to clipboard with ctrl+o and when getting a ref name for diffing.

This commit gets us a little deeper into that hole by explicitly requiring
list items to implement that method so that we can easily use the new
helper functions in list_controller_trait.go.

In future we need to just remove the whole ID thing entirely but I'm too
lazy to do that right now.

pkg/gui/controllers/branches_controller.go

@@ -33,6 +33,7 @@ func NewBranchesController(
 			c,
 			c.Contexts().Branches,
 			c.Contexts().Branches.GetSelected,
+			c.Contexts().Branches.GetSelectedItems,
 		),
 	}
 }