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/switch_to_diff_files_controller.go

@@ -36,6 +36,9 @@ func NewSwitchToDiffFilesController(
 			c,
 			context,
 			context.GetSelectedRef,
+			func() ([]types.Ref, int, int) {
+				panic("Not implemented")
+			},
 		),
 		c:                c,
 		context:          context,